Leave them on, then you know if they still work.
Also quite like having a avionics master then you can leave them all switched on thus stopping the wear and tear of the on/off switchs getting fannyed about with every trip. I don't know if this is a good idea though as you might get funny eddie currents getting at the avionics back through the earth.
Must admit my ex CFI used to say leave the pitot heater off because it burns out the element. Never heard of a pitot heater burning out yet on a light aircraft and the engineers said it was a pile of !!!!e as well so you might as well have it on. Better to have it on than think later bugger i wish i had turned it on earlier.