Anyway, here are some serious reasons to stop healing someone.
A disconnected player
Let me share a story to help illustrate my point. It was a Monday night. We had 15 minutes left before the raid was going to be called. We were staring down the Lich King himself. My guild fought furiously past his Shambling Horrors, his annoying Val'kyrs; we sidestepped the Raging Spirits he summoned. When we hit the last phase with the Vile Spirits, we were able to skillfully dodge his Defiles.
Except for a lone mage.
That lone mage was the target of a Defile. The rest of the raid moved away from him, but he stood there in the puddle all by himself. I started healing him, trying to give him some time to get out. After a few seconds, I realized he really wasn't doing anything. He must have disconnected. So I did the only logical thing I could think of to contain the Defile: I immediately commanded my healers to stop healing him altogether. He fell over moments later, and the crisis was over. (Until our bear tank backed up and fell over the ledge. Thank goodness the other tank was quick enough on the taunt.)
In most cases, disconnected players won't need the healing. They're as good as dead until they get back online, and there is no guarantee of that happening unless you hear them say that they're attempting to reconnect.
Like most rules, there are some exceptions. If the disconnected player happens to be the tank and has a good amount of threat, you just might be able to salvage it until things stabilize again.
Way out of range
Did one of your players stray way out of range? Not much you can do unless you want to chase after them to help preserve them. If your raid is one side of the area and Millhouse the mage happens to be on the other, he might be dead by the time you get into range, anyway. You may as well just not worry about it and let him go. If the opportunity presents itself, pop a Rebirth.
A called wipe
This is the most common reason to stop healing. The raid leader calls a wipe and wants to restart the encounter again with a fresh crew. At this point, stop healing the tanks, stop healing yourself and stop healing anyone else. Go run into the closest fire.
Someone else needs the healing
The last reason is one of those micromanagement-type decisions in which you're given the option of healing either A or B. A must be kept alive, but B is the one who is in imminent danger. But you know that if you switch off of A, A is going to die. Why? Who knows. Maybe you're the only healer left alive after the rest of them died for some freak reason, and the raid leader decided to keep going.
If that happens, then guess what? You're the last line of defense! Choose wisely, and pray that the rest of the surviving members can do their best to stay alive!
Generally, this is considered bad practice to rely on a person's dying and using a Soulstone or a Rebirth to revive them again. I'll only mention it because it is an option, for now. I remember reading in a blue post somewhere that this was something that the developers did not want to encourage.
A great encounter that helps illustrate this example is Professor Putricide. If your raid reaches phase 3, one of your members will be afflicted with Unbound Plague. Instead of going through the motions of bouncing and rotating the plague, you might be better off sentencing that player to his doom and relying on the brute force of the rest of your raid to take down the professor.
Again, that's a last option to be considered ... but it is still an option.
Remember, you still have Rebirth and Soulstones as tools to help with raid deaths if you need them. Don't be like me and forget that your raid has warlocks. Or that warlocks exist. Abuse their stones!
Need advice on how to work with the healers in your guild? Raid Rx has you covered with all there is to know! If you have specific questions about healing in raids or guild groups, email firstname.lastname@example.org -- you could see a future post addressing your question. Looking for less healer-centric raiding advice? Take a look at our raiding column, Ready Check