I play either role in dungeons. If I play healer, I'll give it to the DPS that impresses me with their numbers or a tank that is a beast with DPS and cooldowns management. As a DPS, my order is Healer that DPS > Amazing tank > Amazing DPS. As a tank it's Healer that DPS > DPS that impressed me. A healer that never DPS will never receive my comm.

However, sometimes you'll get paired up with pre-made 2/4 or 3/4 groups, since they can't commendate to each other, they are most likely going to commendate YOU if you did a good job or your job.