First off, reboot and reformat are two completely different things.
Reboot = restart, or shutdown and start back up
Reformat = wipe the hard drive and install windows.
When should you reformat? When the computer will not run properly anymore and you are too lazy (or don't have the time) to fix it. Or if you like doing that sort of thing, then whenever you get bored. Me personally, I have better things to do.
I vote leave it be...