I leave my computer on but thats because my screen saver is http://boinc.berkeley.edu/ it's a program that allows you to donate your computer time to distributed computing research. Cancer, SETI, Hunger, Diseases, Natural disasters, basic science. You get to choose what programs you support. I've been running some form of this on my computers for over 14 years and I've calculated the cost in $$$$ for electricity is about $90-110 per year. Something I'm happy to donate to research. So no I don't turn off my PC, but I think I've a good reason not to. Much like Sony's playstation folding@home I think this is a good use for my PC when I'm not running it.