It costs a fortune to leave a PC running 24/7.
It does vary a lot with the machine and it is being continually improved. Putting the computer into sleep mode when you are not using it does help a lot but turning it off is the go. Besides, as maidup pointed out, they do need to be rebooted from time to time.
Incidentally, Intel is gearing up for a new technology that promises to cut the core power consumption in half. Of course this advantage will be used to increase the density on the chip so the reduction won't be so big.
http://www.pcmag.com/...0,2817,2384909,00.asp