no offense BUT there is no ''right'' and ''wrong'' there is 1. personal preference and 2. not all trash packs are the same.
Personally i like the dps attacking what i am attacking.. it makes it harder for them to pull off me ( they still do though ... no one bothers to drop threat during trash )
To make it easier for them use either verbal calls or marks .. for example bind flame on a easy key and mark your current kill target with it .. when it drops mark something else ..etc..
If the DPS start whacking random targets they'd better be geared enough to take the damage.. especially so in random LFG groups.
In actual ops I don't even try to tank trash anymore as no one bothers to follow any type of a kill order
and to be fair trash in the new instances is so much easier than in EV and KP ( especially in 16 man )