Something I've noticed is that during 4man dungeons the tank is usually more likely to get more commendations, and during 8man ones the healers are more likely to get more. lol just from my experience from being a tank and my healer friends. Though overall the one that best communicates/most positive (or is most helpful especially to first timers) is also one of the biggest factors on who gets the most commendations over everything given they don't play terribly to offset that haha.