If I queue for a duty as a tank, I usually give the healer a commendation.

If I queue for a duty as a healer, I usually give the tank a commendation.

If I queue for a duty as a DPS, I usually give a DPS a commendation.

The only exceptions are if a particular player stood out as exceptional, or I'm queuing with a friend and need to throw the commendation somewhere random.