I main a healer so personally if it's a group with more then one healer i'll give the vote to my healing buddy. If I'm a single healer i'll give it too the tank (unless he's a crappy tank in which case i'll give it too the most outstanding dps). If you main a dps give the vote to however you think did the best job usually it's the healer who saved your butt when you're about to die, or the tank who pulled agro off you, or even a dps.

The point is pick whoever you think left a positive impression on you, if they knew the run, were helpful, kind, give them the vote.

Dunno if this helped at all.