On one side of the coin, a PC that's shut off consumes no power. On the other side, too many on/off cycles means increased wear and tear. Patty Harshbarger at the Bradenton Herald went looking for a definitive answer and found this:

So what is the most energy-conscious routine for computer users? Certainly shutting a computer down, then powering up again within two or three hours would not be cost-effective, and might cause undue wear on the components. But if energy conservation is your main concern, Florida Power and Light recommends shutting down each night.

FPL reports that office equipment represents approximately 7 percent of home energy consumption. During use, Energy Star settings should be maximized, which can save approximately $20 per year for each computer/monitor system. Access these settings by right-clicking on your desktop, selecting Properties, then selecting the Screensaver tab, then "Power." Here you will find a series of pages on energy consumption. When making adjustments without tech support, take care that you understand the effects of your choices.

I've suffered no ill-effects shutting my PCs down at night. And I usually upgrade way before hardware failure becomes an issue, so I'm not too concerned. What's your strategy?