I always queue as scholar when queuing alone for the reduced queue times. I generally get commendations when my group is not so good and I have to do a lot more obvious work to keep everyone alive. When the group is all very well geared and knows the fights cold, I find that I get the commendations less often, which makes sense.

As a healer I usually give my commendations based on what happened in the run. If the tank never let any trash beat on me and kept aggro really well I'll give it to them. If one of the DPS manages to avoid all or most of the avoidable AE damage in a fight (while still doing lots of damage) I'll commend them.