I used to leave it on 24/7, but after moving, I turn it off everynight, or if I'm going out for an extended period of time. It does serve as a heater during the wintertime, but it doesn't really get that cold here in Cali.
I read somewhere that even if some of your electronic stuff are turned off, as long as they're plugged in, they might still be drawing minimal power from the mains, so you might want to switch off your surge protector as well -- that's if you really want to save on electricity 🙂