As pointed out, in some environments, PCs must be left on to receive patches and updates. As more and better power-saving features are built into the BIOS and/or OS, and as PSU efficiency improves, the issue of waste should diminish considerably. So leave it on.
Thermal cycling might eventually work the RAM chips or even the CPU up out of their sockets on the original IBM PC, and other similar models of that "era," but this has also become a non-issue. So turn it off.
Which way do you go? I'm all for being more green, but I don't think this discussion is the place to spend the effort. Go get CFL bulbs. Turn your thermostat down in winter and up in summer. Insulate your attic. Walk a block or two each way, or at least combine all those little trips into one. There are so many better ways to expend the same amount of effort to save energy and reduce waste.