I was taught that turning off the power completely actually causes some damage to the comp because your sending a shock through it every time you restart it. I never "hurt" my comp by doing this... so i just turn it off. Besides, better newer mobo's still have power going through them even when it's powered down as long as it's plugged in.
Turning the computer on and off does cause damage, VERY little damage. As proof, I have computers from 10 years ago that can still run today on the same equipment, so it proves that you save money. Trust me, in 5 years, you'll realize that the rig will still run whether you turn it off or not. Then you'll see how much money you've wasted leaving it on.