Only good reason and that's where most of mine come from.
UPDATE: I would dare say that my rep as being genuinely funny has by and large insulated me from this "funny abuse" problem that we seem to be having. I have in effect not really left people a good option when they dislike a post since I WANT funny votes, and maybe that's the funniest thing of all. Feel free to hit that comment section, kids. It looks like that's your only option if you're pissed at me and even that hasn't proven to be an effective tool since it allows for the legendary numbah counter comments...