I have given commendations to dps, healers, and tanks.
For me, it really all depends on how each person in the party does their job and whether they perform well in the dungeon.
If everyone performs exceptionally, at that point it goes to who was the most polite or was the most useful in party chat by providing strats or stuff like that.

My biggest problem is that people tend to just leave immediately and not even stick around long enough to receive commendations, let alone give them.
It has been so bad that I have gotten in the habit of reminding people to please do commendations before leaving.