There are always exceptions, but I don't usually commend tanks and healers, I try to commend the DPS - especially if they are sprouts - choosing who I think did the best job.

And that's not commending the highest DPS either, it can be their positioning, how they deal with mechanics, if they are friendly in chat, if they put in the effort, and so on.

For me it's about encouraging good gameplay and gently guiding people into those good habits, rather than rewarding a geared level 80 for a smooth run.