From a damage dealer's perspective, my guess would be that the tank & healer are already going to have the "Commend Each Other Circle Jerk". And, often, it's true. Because of this, I tend to give my commendation to a damage dealer. Unless they have really made a mess of things, then I look to the tank/healer (favoring the healer due to them having longer wait times than tanks).

Sometimes I will give a commendation to a new tank/healer as a means to encourage them to continue playing that role.

But if everyone did great, then I get conflicted and wish that I could give more commendations.