I'm getting tired of the collection quests, so any time one annoys me too much the first time, I do it only on my main and perhaps one other toon. (My main is my hunter - the highest damage outputter in the group.)

I'd rather have a dps be the highest level than a tank. A tank won't help you clear instances faster if they can already handle the place - but a dps will. (Note that a healer also won't help you clear it faster.)