I left my old computer on 24 hours a day for about 4 years, minus a few days here and there, and it's still working (minus the hard drive). HDs disintegrate over time anyways, so who cares. I had one computer that would spin the CD in the CDROM drive nonstop no matter what, so if yours does that it might not be too good an idea to leave it on. In the summer my room gets insanely hot, so I turn off my computer when I'm not using it, so as not to melt the thing. I think it turns itself off once it hits a certain temperature anyways, but I'd rather not obliterate my HD partitions in that particular way.

So in summary do whatever you want. By the time leaving your computer running actually manages to add up to real harm, it'll be so old that it'll only be worth $27 anyways.