Well, if you eave it on, the bill rises, yes, but also, it may annoy you during sleep time if it's loud.
Turning it off and on like an average user will eventually kill the PSU, but that will take ages. Heck, a Pentium 1, no...My friend's 286 was being used like it. whenI picked it up...it was still working. IMO, it's best fit.