depends on the pull.
if there are 2 elites, 3 stongs and 4 weak, then 2 weak going after dps/heals is going to happen. if you are an off tank, its your job to pick those up and try to get threat on ones the dps are attacking that the main tank isnt taking care of. any mob that isnt being attacked by one of the tanks is most likey going to get on a healer.
Dip's pretty much right on, but if you have dps that don't follow kill order it makes life a pain in the butt. Their is maybe 2/3 pulls in the game where it isn't possible for the tank to agro every mob in a pull. Just focus on keeping threat on all the stronger targets. If you get to the end of pulls and the last thing alive is a normal mob, you just have ignorant dps and it's really not your problem. If it's becoming a problem with the healer or dps taking a lot of DMG and they begin to say you're at fault, just tell them weak>strong im doing my job could you please do yours. Those weak mobs die in 1/2 GCD and should be the first things dead. Nothing is more irritating then jumping in throwing an attack or two on a gold while moving and rounding up all the weaker mobs and the gold takes off towards the dps, thinking just cause he has some gear he can kill the big boy first.That being said don't have tunnel vision and don't become ignorant yourself.
Also make sure you don't blow your taunts right off the bat, leap/sprint in, throw out a couple AoE attacks then use your taunts.