The rules, as they were explained to me shortly after commendations were released:

The tank always comms the healer, and the healer always comms the tank.
The two DPS comm each other.

That way everyone gets a commendation.

The exceptions are:
- Did someone perform a fight saving miracle? (e.g. kited a boss from 5% to death.) Give them all the comms.
- Did someone do a jobs so bad you were tempted to abandon the dungeon? Don't give them a comm.