I give my commendation to the most helpful and/or amusing player (via chat). If the team wasn't needing help, I give it to a random DPS.
I rarely commend the tank unless, ironically, they screw up and then draw aggro in a situation where it was really needed. It's a tank. Just like a healer, they get too many commends for just doing their duty. Nobody appreciates the DPS roles. :|