I always commend friendly, chatty people regardless of role, and generally default to commending miqo'te in quiet groups ever since everyone switched to au'ra. I don't tend to commend healers unless I notice that people are taking a lot of unnecessary damage. Whether or not they DPS doesn't tend to impact things much (though honestly I probably commend healers that focus on DPS less often since in my experience in DF, they slip up and people die a little more often, which makes the run less smooth).
Edit: And I give commendations to new players most every time I'm paired with one.