I don't reboot unless Software Update forces me to. Even then, I usually procrastinate the update so I don't have to reboot. My MBA currently has uptime of 61 days so...
One more vote for only when necessary. I have Leopard on my iMac so there is never much to update anymore so it hardly ever gets a reboot. I still need to update Quick Time but haven't gotten around to it yet since it needs a reboot for that. Really no reason to reboot unless you have a problem or software update.