Blizzard's Rolling Tool: Use it
Blizzard's rolling tool has come a long, long way. At this point, the tool knows whether your class can use a piece of gear, and the tool allows you to roll need, greed, or disenchant. Just in case there's some horrible error, you can even trade gear among members of the instance for some time after the roll, unless it's been disenchanted, of course. (You only get the disenchant option if you have a disenchanter in the group.)
As a general rule, you should use the rolling tool exactly as it is. Don't try and modify it with freaky rules and regulations. Just click need, greed, or disenchant, and move on with your life.
The tool does have a few challenges. We'll discuss them in just a second, but I want to underline: Don't do anything but use the tool as intended without discussing it with the group first. In the attempt to be a good person regarding gear, you could very easily just screw yourself out of loot. No sense in trying to alter the rules if the group doesn't agree to it beforehand.
Consideration in low-level dungeons
One of the places the rolling tool falls a bit short is in lower-level dungeons. A few natural situation illustrate why. First, groups are comprised of people leveling up through the game, and most characters won't yet be settled into their final role. Along the way, tanks and healers especially will be trying to maintain two sets of gear: one for DPS and leveling, one for completing dungeons. Second, some group members will be trying to switch roles, collecting DPS gear while still tanking the instance you're currently in.
While that dynamic shouldn't give a person priority in a need role, it's fair to say that's a good reason not to disenchant an item that would otherwise see use. Most players agree that disenchanting is cool, but it's better for a piece of gear to actually get used. Breaking a useful drop into a shard is kind of a putz move.
For this reason, check in with the group members to see if anyone might otherwise use gear before you start rolling disenchant. This should only take a second or three and can save some heartache for someone who's trying to leave DPS behind to get into the tanking game.
The Raid Finder: No exception
Blizzard just recently changed the Raid Finder loot system to refine who can roll on the gear and to prevent folks from winning multiples of the same item in a single roll. These changes fix an etiquette issue that had been lingering in Raid Finder: People roll need on everything they can use, always, no matter what.
I'd like to be polite and say, "Just roll until you win one item" and "Maybe help out your fellow gamer." The problem with this approach is that you're in a Raid Finder with 24 other people, and they probably won't be nearly so considerate.
Use a little judgment if you have lucky dice. If you've won three new pieces of gear, it's OK to start passing on rolls. But just because you won one sidegrade, don't just stop rolling. Blizzard created an awesome system in Raid Finder; if the designers want you to only win one item in a run, they'll change the system again.
Off-spec and off-type armor
Once you're cruising around the raiding content, you'll never wear cloth caster gear if you could be wearing leather or plate. That's not true in the lower levels. At level 20, good stats are good stats, and the exact class of armor be damned.
What do you do if you're an elemental shaman who could roll on that cloth that just dropped? If there's no competition in the group for the piece, roll on it and have a good day. If you'd be yanking it from a priest or mage, though, then skip it.
Yes, I know caster leather, mail, and plate is a little rare in the lower levels. But the armor specializations (which give you bonuses for wearing the right gear) are in the game for a reason: to encourage you to wear the right class gear.
The world won't break asunder just because a balance druid is rocking a cloth hat. At the same time, there's no reason to start picking fights in lower levels by yoinking other classes's gear. This awkward phase passes as soon as you get into later content, and it's just not worth the fight.
My final tip is my pet preference and peeve: If someone wins gear -- especially if they beat your roll to win it -- then congratulate them. It's simple good sportmanship.
Visit the WoW Rookie Guide for links to everything you need to get started as a new player, from how to control your character and camera angles when you're just starting out, to learning how to tank, getting up to speed for heroics and even how to win Tol Barad.