In FFXI and WoW the rule was simple.

Give about 5 to 10 seconds at the start of the fight for the tank to gain hate then start dps.
In this game you don't need 5 - 10 seconds to build up an aggro lead. :-p I support letting the tank pull and take a moment to position mobs, but you do not need a 5 - 1 0 second lead.