My main class is DRG, but usually if I'm playing an instance on that role, I'm part of a pre-made group, so I cannot give out commendations. Though one time when I was doing DR Praetorium, I'm pretty sure the other Dragoon and I commended each other. Only commendation I've received as a DRG. I also got one as a THM. The rest of mine came as a CNJ/WHM.

Playing as a healer who mains a DPS class, I usually pay attention to what the DPSers are doing. Especially if they're LNC/DRG. Do they keep Heavy Thrust up as much as possible? Attack the right targets? Avoid AoEs? Chances are they'll get my vote. Harder to judge the ranged DPS. If none of them stand out, though, I'll give my vote to the tank, assuming he/she wasn't an idiot.

Also add me to the list who wants to be able to bestow commendations while in a preformed group. I ran WP as a healer for the first time the other day with one of my FC buddies. Awesome tank. I told him at the end that I would have commended him if I couuld.