I've always been told that turning off your computer every time you don't intend on using it for a while is not the best idea. Switching it on and off will do more harm than it will do good, your pc's power supply will be worn down very fast (it's just like a striplight really

).
I usually turn on my computer when I get up and turn it off when I go to bed at night. I only turn it off when I'm going away for long during the day, because I'm a little bit afraid of it short-circuiting and causing a fire in my absence. I may also turn it off because of the noise it makes. My power bill is divided between all 500 people in my apartment building, so I can't tell the difference I guess.
In the end I think it comes down to balancing the pros and cons, the higher power bill or the worn down pc...