Hmm. My approach is usually this: If I'm tanking, and someone pulls aggro by accident, of course I grab it. If they managed to get it from me, then I grab it, and often apologize. It is not hard to get or keep aggro as a tank, and if I lost it, its basically my bad.

BUT. If someone else intentionally pulls a group we're not fighting without even saying anything, it is disrespectful. Yeah, tanking is easy. Yeah I could grab them off. And, the first time, I do. And then I explain that if they want to pull again, they will have to deal with it. They could have asked me to go faster. They could have talked to me. But if they can't be bothered to just treat me like a human and speak to me, then I don't really care if they get mad or kick me. I'm a tank, I have an instant requeue. It's the shrug heard 'round the world at that point. But there's no excuse for being rude when you could take 10 seconds to ask if the tank will go faster before taking their job on yourself. ^_^