I rarely pay enough attention to who's doing what to really care at all. I'll notice bad players far more than good.

Tanks might get a comm if the run was smooth and swift, owing to their skill through trash pulls.
Healers might get a comm if I noticed they had to put up with something awful the entire run.
DPS might get a comm if I felt everything died especially fast.

If I didn't notice anything particularly good or bad, average run of little note, it just goes to someone random.
If the run is really bad, and I notice everyone failing at one thing or another, nobody gets a comm.

But I also don't have the comm message in my usual open chat window, so I don't ever pay attention to when I get any of them. Watching to see if you actually get any seems like it puts too much importance on them. They're meaningless and not worth noticing.