I'll usually just throw one out to whoever at the end. I've been giving it to the DPS more often recently just because they don't get all that many otherwise, but there's no rhyme or reason unless the tank and healer were bad.

Sometimes I'll do it just because I like the class they're playing, their glam, their name, whatever. If they're new I'll probably give them one just because. But generally I just throw it out as I wait for the loot or am walking to the exit.