If I'm tank I comm the healer.
If I'm healer I comm the tank.
If I'm DPS I commend the other DPS.

Someone has to be pretty obnoxious for me to not give them the commendation.

As far as I can recall no one has ever directly asked me to give them a commendation, I wouldn't not give it them, but I also doubt I'd change from my normal system either.