Leave it on. It isn't so much the ons and offs as the thermal cycling as the whole megillah cools down to room temperature and then warms up to operating temperature again. It makes socketed components creep loose among other things. Any old hands who have magically cured hiccoughs by reseating DIMMs and BIOS chips will know what I mean.
I leave 'em all on unless I'm going to be away for a long time and they're often up for days (if the M$ ones don't crash that is). I've added an extra cooling fan to all of 'em too, even though only one is a bit overclocked.
I've only had one PSU die on me over the years, but several PSU's have needed new fans. Not difficult to replace yourself, but get a ball-bearing fan rather than the commoner ones with a sleeve bearing - lasts far longer.