Unless the group as a whole just really annoys me, I always toss a commend to someone.

Usually a DPS.

I notice people seem to toss commends to healers and tanks just by default - as if the two easiest roles in the game needed a special gold-star just for showing up... (I have mained tanks and healers in MMOs since 2005 - and yeah, there's not a single MMO out there where the hardest tank or healer to play isn't easier than the average DPS, even here where part of the task is to also maximize DPS, the rotations for that are even easier than a redmage).

If I can find a leaf, and they played even up to "OK" they're the likely target of my commendation. Especially if they did something to show a good attitude.
- otherwise it tends to be somewhat random through I tend to disfavor giving it to a redmage or a samurai as those two are super-easy...

I only give it to a tank or healer if they do something unusually good or show a particularly helpful attitude or are the only green-leaf in the group.