I normally stick with my own role.
If I go to the dungeon as DPS - I will commend other DPS.
Though I main Tanks so in general I would commend Healers (in 4 man dungeons at least). With exception of situations when Healer is really bad or when I went with Healer in premade party, then I will commend DPS.
I hardly can remember any situations when I don't give commendation at all, though. If everyone play bad - well, maybe then I will go out without giving commends, but... what if problem was in me? xD