If someone did something noticeably good, obviously they get the commendation regardless of class. Similarly, if someone was friendly & fun during the duty then I'll give it to them regardless of class.
When I'm DRG I tend to give it to other dps because I know the commendations come less often for dps.
When I heal, it's usually the tank, especially if they're efficient in pulling and can take a hit while I lay down some dots.
When I tank, it's usually the healer, or possibly the melee dps if I see them doing their job well.
All of those are subject to people being excluded for screwing up.
Also, as a side note, sometimes I wish we could give out "condemnations" or the like for when someone really pisses me off.