Magmaw Preferred spec? Either, though holy might be a tiny bit stronger here. Power Word: Barrier is almost useless on this fight if you use a spread strat (not all guilds do). Power Word: Shield is good for keeping low-health targets from dying to random Magma Spit, though. Holy priests have a slight mana advantage over disc priests when Magmaw is spiked, since they recover more mana while standing around doing nothing than disc priests.
What's different? More adds and splash shadow damage on random targets sub-30%. This doesn't make a big difference for healing if your raid spreads out properly. All the same, plan to use abilities like Divine Hymn after 30% to help keep your raid up. Lightwell is great, since players are spread out. If you're really having trouble, try to arrange your spread so that players in the same group are standing closer together. This will ensure that you don't lose range on some players with Prayer of Healing.
Omnotron Defense System Preferred spec? Either. There is plenty of raid damage, both AoE and single-target, and having one of each priest type is ideal for dealing with all the different kinds of damage.
What's different? Each golem has received new abilities in its repertoire. The victim of Acquiring Target will be hit harder, which means you'll want to use a tank cooldown like Guardian Spirit or Pain Suppression on him. Raid damaging abilities are obviously harder to deal with, but the one to use Power Word: Barrier on is Shadow Conductor. Incineration Security Measure isn't nearly as devastating, and a Holy Word: Sanctuary works out quite nicely on it. Priests of both specs should also keep in mind that you can save time by using Leap of Faith on the target with Shadow Conductor to get him back into the raid sooner.
Maloriak Preferred spec? Either. Holy is strong during the black phase for healing the raid damage, while disc could almost be considered mandatory for Scorching Blast on the red phase, since the damage is now quite strong.
What's different? A new phase, the black phase, requires a lot of raid healing, since the Dark Sludge spawned by the Vile Swill isn't always avoidable by melee. That's good for holy priests and spot healing disc priests. A new ability in this phase, Engulfing Darkness, is channeled on the main tank and reduces healing. A disc priest shield is very helpful to get the tank through the duration of the channel. Lightwell is still great on blue phase.
Atramedes Preferred spec? Holy, though Atramedes is very popular for discipline priests trying to shield spam their way to a record healing parse on World of Logs. There is a lot more random raid damage in the fight than anything else, which means holy shines. Being able to move around faster with Body and Soul is also helpful when you need to dodge Sonar Pulse. You can dodge them no problem without Body and Soul, but the speed bonus means you can get where you're going faster and start healing again sooner.
What's different? Almost nothing. To date, the fight has been nerfed so many times that the differences between heroic and normal for healers is insignificant. Making sure your raid is topped off before Searing Flame is a little bit more important, but even if half the DPS die during the span of the encounter, you should be able to down the boss. (You can use Power Word: Barrier on Searing Flame, too, but that would imply you went disc for this fight, silly.)
Chimaeron Preferred spec? Discipline. The fight isn't dramatically harder on normal if your healers check their egos and work together. Disc is ideal for spamming Power Word: Shield in phase 2 (when players become immune to healing), but a holy priest specced into Soul Warding is plenty effective.
What's different? The major difference is that when Finkle's Bile-O-Tron robot is knocked offline, Chimaeron will attack the tank instead of fighting itself. Plan to use a tank cooldown, Guardian Spirit, or Pain Suppression whenever the bot is down. You can also use Barrier and Holy Word: Sanctuary to deal with the increased damage from Caustic Slime while the bot is offline.
Nefarian Preferred spec? Discipline. The increased heroic damage makes reoccurring abilities like Electrocute extremely dangerous to your raid's well-being. Disc will be strong because of Power Word: Barrier, which takes the edge off Electrocute. After that, disc single-target healing is preferred because raid damage is relatively light (outside of Electrocute) and because most of the time, your raid is separated into smaller-sized groups.
What's different? Explosive Cinders in phase 2 will require players to temporarily leave their platforms so as to not kill your group with the detonation. These players will require hard healing but nothing unmanageable. You'll want to use Pain Suppression (or Guardian Spirit if you choose not to follow my advice) when you or a fellow healer on the same platform gets Explosive Cinders. This will help to ease the amount of extra healing required from Explosive Cinders when a healer is preoccupied. I would recommend grabbing Desperate Prayer in the holy tree for this fight in case you have to jump in the lava more than once.
Throne of the Four Winds
The Conclave of Wind Preferred spec? Either. In general, this fight isn't about what spells or abilities you have in your toolbox, just that you use some sort of healing spell at the right time in the right place.
What's different? There are new abilities in the fight, but nothing that will dramatically change the way you heal. Just focus on being where you're supposed to be and dodging Wind Blast if you end up on Rohash's platform.
Al'akir Preferred spec? Holy. Serendipity is strong on most fights, but on this fight in particular, being able to heal bomb a player with a speedy Greater Heal is almost essential to surviving the duration of Lightning Strike. Your single-target Chakra shines here because of Holy Word: Sanctuary, which beats out Penance by being instant. Being able to sprint with Body and Soul is also great for moving in for Wind Bursts and out for Squall Lines. You'll want to grab Soul Warding so you can have a little bit more healing during Lightning Strike than you would normally.
What's different? Lightning Strike, as I already mentioned, is especially volatile because of its 30-second duration. Nothing else should stand out for healers though.
The Bastion of Twilight
Halfus Wyrmbreaker Preferred spec? Discipline, though not because of Smite spamming (that has ceased to be necessary or viable like it once was.) Instead, you want to use disc because there are so many tanks required for the fight, and a Power Word: Barrier can be used to apply a cooldown to all of them at once. Pain Suppression and Guardian Spirit are handy here as well.
What's different? Having all the drakes out at once means tank damage is intense at times. You can deal with this like you usually do as disc or holy, but try to stay ahead so you don't end up having to use your cooldowns outside of when the tanks call for them.
Valiona and Theralion Preferred spec? Either. Disc is good for placing Power Word: Barrier when your group gathers up for Blackout or to port down to the twilight realm. Holy priests are good at keeping the raid heals going while dodging the various stuff on the ground. Holy will want to shy away from spamming HoTs, though, in case they get targeted by Engulfing Magic.
What's different? A shadowy twilight realm was added to the fight, but nothing about it is specific to priests. There is still plenty to heal, though.
Ascendant Council Preferred spec? Holy. The sheer amount of random raid damage is best tackled by holy priests. Body and Soul is extremely useful for getting to a Gravity Well or tornado (Call Winds).
What's different? There are extra abilities added to the first two phases. Gravity Core in phase 1 is great for priests of either spec. Simply have your priest stand outside the raid stack and use Life Grip on the player who receives the debuff. This will ensure the ability doesn't tick inside the raid. In phase 2, Leap of Faith is also useful for grabbing players to an ideal location to kite frozen orbs through flame patches.
Cho'gall Preferred spec? Holy, for the same reasons I stated in my original guide to Cho'gall. The ability to switch between Chakra stances lines up perfectly with the cooldown on Chakra (something we still need to be mindful of, even when the duration is unlimited). Disc priests can be useful, however, if your raid decides to force one player to Corruption: Absolute. Fully corrupted players are immune to healing, but they can still be shielded.
What's different? Unfortunately, very little. I guess the developers thought the normal version of this fight was interesting enough for healers already, that raising the tuning on it all was enough.
Sinestra Preferred spec: Holy, because of how you can use Guardian Spirit on your first Wrack target. By dispelling Wrack later, you make all the derivative dispelling easier, so if you Guardian Spirit a player who is about to take a lethal tick of the first application of Wrack, you'll be buying yourself more time later.
What's different? Wait a minute ... There is no normal mode! If you've made it this far, congratulations. Your talent and hard work that has gotten you to this point should get you through this fight. Prayer of Healing is ridiculously good on this fight, particularly before Flame Breath. If you learn to time that up right, you'll be in good shape. Do keep in mind that in phase 3 Essence of the Red will eventually wear off. Healing will get very intense at that time, so plan your mana regen cooldowns around that part of the fight so you'll have enough mana to heal the damage.
Spiritual Guidance has the inside line on pre-raid gear, valor point and raid gear, and healing strategies for bosses such as Atramedes and Chimaeron. Newcomer to the priest class? Look into Discipline Priest 101 and Holy Priest 101.