I'm a relatively new player, but for what it's worth, I've just always thought it was both common courtesy and common sense to let the tank lead. Not just for the sake of a smooth run, but out of consideration for the tank, who may or may not be comfortable enough with a duty for wall-to-wall pulls (no matter how easy some players may find the content in question, everyone starts somewhere).

That said, I think if a tank stonewalls the party for daring to pull ahead, that's tantamount to griefing. Of course I also believe whoever pulled first should lead the mob to their tank if they do insist on running ahead, but all things being equal, everyone should cooperate as best they can.