As tank, I tend to give it to the healer as long as they weren't awful. As healer, I tend to give it to the tank as long as they weren't awful. As DPS, I tend to give it to the other DPS as long as they weren't awful. In eight man content, I'll commend my counterpart as long as they weren't awful. Are we seeing the pattern here? xD

If my "usual" was awful, then I'll throw it to a random DPS (as long as they weren't awful) because I know it's tough to pick up commendations as a DPS. If there's someone else that did something memorable, they get it. The healer that bails someone out with Benediction/Lustrate. The DPS that LB3's at a crucial moment. That kind of thing. If the group was 100% awful I have no problem with not giving a commendation at all.

A lot of the time I queue in a partial party (usually 2 or 3 of us from my FC) so I'm sure there's a lot of random DPS wandering around out there wondering what they did to get 3 commendations. xD