Re:Turning off PC" = Good/Bad
The theory is that by leaving the computer running, the components stay at operating temperature with constant current. Turning a computer on involves sending an initial surge of current to each part, then it warms up. Most electrical appliances fail at startup because of this surge.
If the computer is turned off after use, the theory says that continued expansion and contraction of parts, due to temperature variations, may cause stress cracks and failure.
Having said all that, I would like to say that I live in Australia, where the power is not reliable enough to leave ANYTHING on if it's not in use. The last glitch saw us supplied with 360V, which destroyed some of my equipment. (My surge-protector exploded)