OFF, If you turn the computer off after use. Can take ages to preload stuff. the more memory to fill the more hd thrashing occurs which slows down execution of other apps.
On If you use the sleep function as it wont have to keep preloading.
I have mine set to just preload boot files as other wise it can spend upto an hour just preloading games I havent fires up for months, mp3's and emulation roms. Preloading dir of mame roms ....it goes on forever
Can look at all the useless stuff its preloading in resource manager.