I turn it off at night, as it has to be in my room, as that is where the phone line is, I will never get to sleep with it on, as it is quite noisy, (4X 33 db fans and heatsink is 42 db, way too noisy, I prefer about 0db to get to sleep (unlikly to happen though) I also have a cold cathode on my case, that is like leaving my main light on.
My pc is when I'm awake unless my mum turns it off when I'm out (I have taught (sp?) her how to turn my pc off when I go out, as she turns it off by the plug.)