You make it sound like giving a DPS a comm, is a punishment for the other two members rather than a reward for the DPS. -_-
I personally by default Comm my partner. Small group Tank comm Healer, Healer comm Tank, DPS comm other DPS. In Large group comm someone else in the role I am currently filling. This is unless someone stands out to me as doing a great job of making the instance better. This could be through outstanding play, or as little as being fun and talkative during the run. Lots of ways to make an instance more enjoyable.