Personal order of priority for giving out commendations:

1) Fellow lalas (represent!)
2) People who were especially helpful
3) People with amusing names/glamour
4) DPS who did good (because they deserve some appreciation after queuing for half an hour)
5) Healer/tank if they didn't already ditch the party as soon as the run ended
6) Anyone else if they happen to be the last person left in the group (might as well give it to someone...)

I don't believe in giving commendations to tanks/healers just for doing their role properly, you should be doing those roles because you want to, not just because you get fast queues and easy commendations, incentivising people too much to play roles they're uninterested in learning how to do properly just leads to an overall decline in the quality of players.