I give my comm to whomever I feel deserved it the most. This may seem obvious but it just seems that everyone just gives it to the tank if they are the healer, the healer if they are the tank, or the other DPS. I heal 99% of the time but that doesn't mean I'll be giving my comm to the tank. I think it's easy to see if your DPS' are actually doing their job properly and to be able to determine who is doing the better job. But it mostly comes down who I find the most enjoyable to play with. I actually find that the more I play the less likely I have been to give it to the tank.