You may compare turning on and off you PC to turning on and off your car. Every time a vehicle is started, parts are worn, uses fuel, etc. If you were getting out of your car to run a 1-2 minute task, it's better to leave the car running....less fuel consumption, deterioration, etc.
Use this sort of scenario when assessing PC usage. If you are using your PC in the AM and then go to work, I would leave it running during the day if 8 hours later you are just going to use it again. But if you leave for a weekend, shut it down to conserve power. In the long run, it's personal preference...
-Mike