I usually power my Power Mac down..there's a rule around my household about turning computers fully off..but that's because we've used Windows for many years. I turn mine on in the morning before school, do some brief updating to my facebook status, check the forums I visit, then the weather, then I hit the sleep mode keyboard shortcut. Though since I use USB extension cables on the keyboard (an old G3 style Bondi Blue Apple Keyboard) and a wireless Logitech mouse plugged in on the keyboard, the only thing that wakes the computer is hitting the power button directly on the computer. I like having a button on the keyboard, but since the extension cable makes up for the short cable that came with the keyboard, can't complain. It's a Mac. I'm glad I own one at least.
I'll add that if we're expecting storms of any kind (especially between March and October) during the day, then the computer gets shut down. I'm a weather geek and guru and therefore, I do make my own predictions as well. Turning the computer off doesn't hurt.