I can see no reason "turning a computer" at the end of the day would do it any harm as long as it was shut down and all cables where disconnected when your doing the turning, is this some sort of Feng Shui question?
:)
Seriously, you will probably get people giving both sides of the story, it's up to you, I leave my computers on 24/7 but thats my way of doing it as I want them to be ready to use whenever, if you don't mind the time they take to start then turn them off, don't reckon it makes a big difference to the life span