A fan is not going to hurt at all. If you get one just make sure its a quiet fan. You can always download a temp program like motherboard monitor and see what its running at. not sure on what fans are quiet but I would say any low rpm fan would do just fine.
If the computer is for someone else then I'd say put the fan in just to be sure that they don't smack you later should something happen. It would probably be fine without the fan, but for such a small cost I personally wouldn't even leave that factor up to chance.
Good quiet fans to look for would be something from Panaflow, Titan, or Vantec if you stick to their Stealth line. Otherwise just avoid high RPM fans if you're not overly concerned about the noise.