I don't think it's cheating, and I'm sure it's done all the time. However, others might be wary of this, particuarly because the purpose of the best answer is to award the person with, well, the best answer. I think that if you truly think you gave the best answer, then sure, what's the problem? But if someone asked a question and you answered with something meaningless ("chicken butt"), then perhaps you'd better vote somebody else's answer instead.