I try to comm the most standout player performance wise, regardless of role, assuming all else is equal. One good way to get my attention though, is to do better DPS than me. Not because I'm all that great by any means, but because it happens far less than I would expect, given I main RDM.

But overall, it's just about who seemed to put the most effort in. If no one in particular stood out performance wise, that's when other random things like chat or whatever come into play. If none of that exists, then yeah I'll probably pick tank/healer for filling roles that helped my DPS queue pop (assuming I'm not tanking or healing).