Ideally you'd assign jobs at the start, and give the other tanks the opportunity to be MT on different bosses instead of just hogging them all yourself.

In practice, I usually go ahead on the principle that if you want a job done right do it yourself. More often than not nobody wants to MT until someone else does, and then it turns into a dick-swinging contest about who can pull aggro off of each other most effectively.

The only thing which especially annoys me is if I'm quite clearly getting into position to pull (e.g. circling around Five Headed Dragon's arena so I can pull him from the back) and someone else lazily pulls the boss directly into everyone stood by the entrance and makes a complete mess of getting them into position.