Interesting thread here. In my experience, having been each of the three roles at some point, there isn't a huge difference in player commendations. DPS tend to get lower, but I'm currently a MCH, and I usually get a commendation even during an 8-man run.

I think people who DPS are more likely to pay attention to the DPS and give them a commendation. When I was only a healer, Lord knows I gave most my attention to the tank. I only really gave a DPS a commendation if they did something outstanding or I noticed that they took advantage of positioning and knew the mechanics/quickly learned the mechanics. While tanking, it normally goes to my healer.

None of this takes into account how friendly the players are, of course.