If I'm tanking, I'll give the healer a com if they didn't let me die. If I'm healing, I'll give the tank a com if I didn't get beat on. If I'm dpsing, I'll give the dps a com, unless either the tank or heals were superb and the other dps kept messing up ( melee not using LB or such )

As of right now, leveling a SMN, I get a fair amount of coms for doing my job properly.