Most everyone's pretty much nailed it here, and I too leave everything on. Like they say when you fly, the most dangerous parts are the take off and landing, so for PCs, the most critical points of failure are power on (and to a lesser extent) power down.
I've got a couple of high draw systems at home, but even they don't take up too much power when I leave them idle all day.
If you're nervous about leaving a system on, invest in a good UPS (APC or TrippLite) - they're worth their weight in gold.