If you leave a PC "fully" on, even when not in use,, it will use a lot of power, and the mechanical life of rotating components will be used up. If you power down or even spin down (powersave) the hard disks too much, their life will end up being determined by the number of start cycles.
In general, if YOU take responsibility for shutting down, then the only form of powersave you should do is the monitor.
If you leave the machine on then you should have it powersave as much as possible when genuinely idle - avoid short-cycling.
For a degree of lightning protection, I'd say it should be switched off or unplugged from the mains socket, though if you leave any other cables connected, better the leave it plugged in to keep the case grounded.
PS. Unless you can absolutely guarantee that nothing memory leaks or introduces any other cumulative deterioration, I'd be wary of relying on comtinual run, with powersave, standby or hibernate - only an actual reboot clears the decks.