I main Healer, but give comms to DPS most of the time. Reason being that, just because you are a Tank/Healer, does not mean you earn it by default.

I see a lot of DPS trying their hardest to be good at what they do, they do not get enough recognition for that in my eyes (I play all classes, thou main Healers),
so I'm familiar with DPS classes rotations and skills, and on fight by fight basis I decide who from the 2 to 4 DPS did the best and give the comms to him.

I will give Comms to Healers and Tanks only if they go above and beyond their normal role. Like a PAL off healing when a healer is dead in a Raid, or a GNB gives Heart of Stone to someone just being resurrected so he does not take fatal damage and so on. Same for Healers, I will give you coms if you Pre shield Tank busters, use rescue to help others, etc.