Here's my rules of thumb for etiquitte:

Pulling and Deciding MT Outside the Heat of Battle

-If someone expresses desire to be MT in the chat before any other Tank does, let them MT.
-Otherwise... always assume that the most geared Tank will want to MT. This often overlaps with the Tank with the most Strength.
-Examine the other Tanks before the first pull. If I'm obviously the most geared, I'll try to take charge. You know, to enforce the previous rule.
-If people are putzing around before any pull while all 24 people are inside the barrier, just go take the boss regardless of Tank gear. However, the second rule applies more often than not and Mr. Geared steps up

While in Battle

-If a DPS starts the fight, every Tank is likely to go ham just so any Tank will have it. If another Tank has obviously taken the lead in this regard, back off.
-If a Tank other than myself takes it and then loses Aggro, I'll relieve them of their duties. Hopefully, it'll be before the squishy dies. The previous rule applies if all other tanks follow my lead.
-If an aggro fight begins and I lose aggro without another Tank using Provoke, I back off immediately.
-If another Tank attempts to provoke a boss off of me, forgive the first time. It is possible that they couldn't tell that the boss was stuck to a Tank. If the Provoke didn't steal the boss from you and it happens again, it is a sign of significant and petty desperation to be MT. If there are no adds or off-tank duties to worry about, it doesn't matter what happens from here. If I'm in no danger of losing threat, I just keep it. Otherwise, leave it alone if the other Tank manages to steal it for more than five seconds or so. If there are off-tank duties, then I'm fighting with a Tank who clearly won't perform them, so I should back off and go do them.
-If I fear for a Tank's survival, I sometimes use a harmless Provoke(Provoke and then don't attack for a few seconds) to ensure that another tank is number 2 on threat in case of a Tank death.