I always turn mine off at night. It has caused no damage so far. You should at least restart it every once in a while to clear out the RAM. I once left mine on for a few days straight, and it really bogged down. I restarted and it was fine. I think it probably wears out faster to leave it on. The hard drive and the fans would probably go out the fastest. Probably the difference is negligible. After a few years of leaving it on for long periods of time, or shutting it down every night, you still need a new computer, simply because technology changes.