My rules for giving out commends:

In a dungeon, if tank is doing huge groups (Which I main SMN so I always demand huge groups /AoEAddict) and the healer is using AoEs (Holy, etc) and the tank is staying alive. I commend the tank. They are are rotating cooldowns enough that the healer is having next to no trouble.

If the healer is having a hard time keeping the tank alive in big pulls, but they are trying, I give the healer the commend. OR I give the healer the commend if I see them save someone's life with Rescue or a last second heal. (This happens quite a bit tbh)

For a DPS, I give it to them if they they are doing their job role. Such as, I despise BLM that don't AoE that much in big pulls. OR if they are lowered geared and they are trying their hardest to stay alive, I will give it to them since they to continue to try.

If all conditions are met, I have a priority system. Healer that goes out of their way to save someone else > Tank that regulates CDs > DPS > Healer that is having a hard time but managing > DPS that is still trying

I think I put more effort into determining who gets the commend than I do the actual dungeon...xD