YMMV, of course, but this is really a matter of personal preference, and circumstances. I have two
computers, a
Powermac G4, and a non-
OEM K6-2/450
PC. I leave both of them on 24/7. First of all, the computers are in the basement, so the noise doesn't bother me when I go to sleep in my room upstairs. Of course, some people actually
like the whirring sound. I slept in a room with a 24/7 computer for near a year and got quite used to it.
Yeah, it puts wear on your components, but really, is it that big of a deal? If you don't buy shoddy parts, they'll last. My Mac Quadra 700, purchased well over 6 years ago, turned on and off all the time, and eventually left on 24/7 for almost a year...and hey, it still runs fine!
I leave my G4 on all the time, as it runs SETI@Home and I just am impatient and don't like waiting for things to boot up. I also like to leave things open so I can come back them later, where I might have forgotten had I not left the computer on.
My PC has no monitor, it runs only FreeBSD and as such, I leave it on. This is partially because it was my first attempt at building a computer, and thus it's not entirely reliable. Sometimes it fails to detect one of the DIMMs on start up. Sometimes it refuses to even enter the BIOS and just makes a loud beeping noise. Basically, it's safer to leave it on than to risk rebooting it all the time. Plus, I like my l33t 60 day uptimes.
Use your computer the way you want to. Computers are hardier machines than most people believe, if they are understood properly. Any difference affected by turning it on and off, or just leaving it running, will be negligable in the scheme of things. And after all, chances are your computer will be obsolete within two or three years anyway, so you might as buy a new one. It's no big deal if the one you have now would die in five years with your current usage habits, since you won't be using it then anyway. Hopefully.