I give mine to someone who stood out to me. If no one "stands out" to me, i give no commendation. I usually do give commendations though. I find myself spreading them out quite evenly. Did the tank hold aggro and pull well? Did he quickly correct mistakes?
Does the DPS hold back the damage in pinch spots to avoid taking aggro if the tank had a weak moment? Does a DPS player stun/silence occasionally on trash?
Does the healer keep us so high on HP that we aren't even worried about damage?
Who dodged mechanics well?
These are the sorts of things that i ponder when giving a commendation.