I usually do it with a co-DPS. Only time I've given it to a tank was when the other DPS was dead. I didn't know so many gave them to tanks. I guess it is plausible good tanks can out DPS average DPS's, if a person is running a parser, I guess they'll know

That said, I have jokingly given it to a healer friend saying "I'll help you boost that healer DPS", but that's about it. But I won't deny nepotism when it comes to grouping with friends, but if they're not playing a DPS, then I'm giving it to somebody else.