I usually turn mine off every night. The room gets very hot with 2 PC's running within it throughout the day, and opening the window doesn't seem to help much.
The only times I leave it on, is if its rendering something complex, and I'd prefer not to watch it do it, plus give it all the cores/memory to use.
I find if I leave my computer on for too long, the speed starts to degrade somehow. Its noticable anyways