I go in as DPS. I hand them out as

Tank gets it if he/she is willing to listen to suggestions when needed and actually tank the boss away from the healer/dps.
Healer get's it if things were a little crazy and he/she kept his/her cool and kept everyone alive.
DPS gets it if the tank/healer don't do anything to me that warrants it.

I also though will give it to someone who actually admits this is their first time in the dungeon, or they don't know the fight and ask for tips. If someone is willing to do that, to me they deserve it because they are trying their best to make sure they don't fail for the group.