I turn my machines off at night, and when I leave the house for more than an hour. Back in the 386/486/Pentium days I used to leave them running 24/7. That was until one of my sisters computer caught fire, luckily she was at home when it happened. Since then, two other people I know have had power supplies catch fire.
It takes my computer 48 seconds to boot. During that time, if you're like me, you sit there, pretty much staring at the BIOS and Windows screens. Maybe you're even mouth-breathing. Done just once a day, that's 4.9 hours of looking stupid, every year. I'll gladly play $120 to take that off my resume.
Somehow I don't think looking stupid has anything to do with watching his computer boot up. I've clocked my machine boot time at 17.46 seconds. I don't even notice it. When I wake up, I punch the power button on the way to the bathroom. It's up and ready to go by the time I get my coffee.
"You can either want something to be true, or you can want the truth. Pick one." - Mr. Diety