I just about always leave a com during a run, unless everyone else was terrible (and my standards for terrible are much more forgiving than most). During a run where no one was terrible or exceptional, I give a com to the DPS if I'm DPS, and to the healer/tank if I'm tank/healer.

Because the game encourages commendations with a weekly challenge log entry and gets people into the habit of leaving them, not every com you receive may be in recognition of a job well done - just a job adequately done.

As for my personal experience when I play DPS, I receive coms pretty regularly, but I'm hesitant to chalk it up to exceptional play. (Or, possibly, I'm much better than I think I am...)