As a BLM as well as a new, most time I give it to healer just because I might have made them suffering additional work, for my unskilled job.
Sometimes healers are in poor condition, or I really have not made obvious mistake, or just because others are more attractive, I may consider others, such as tanks who pull steadily or players who give useful advice/encouragement to the news.
Anyway, I do not know so much about who "play well", so it's unnecessary to trust my judgement...

And one more thing, when I play the dps role (my second job is tank), I really don't care if I receive a commendation.