Treating the computer like a microwave and flipping the switch a dozen times a day could cause problems. But turning it on in the morning and off at night? Not a big deal.

Either way, it's still a controversy. Turning it off at night yet with two monitors and three hard drives, you end burning energy even with power saving settings.
 
Free Tool to Save Energy
UniBlue's free Local Cooling program. The downloadable software gives you an easy way to control the power consumption of a PC. Once loaded, you can set the tool to switch off the monitor, spin down the hard drives, and hibernate to PC after a specific time.

This tool allows you to see the amount of energy you're saving. After a short while, you can view your estimated savings 0.139 trees, 1.265 gallons of gas, and 2.63 Kwh of power. The site claims over 11,000 users have joined.
Granted, the power saving estimates is just that--estimates. But it keeps your attention focused on saving energy and possibly helps to decrease global warming.

Clean and Tidy PCs
You're not going to save much energy by keeping the insides of your PC clean. But you can definitely increase its life span by getting the dust (technically known as schmutz) off the fans and removing crud from the CPU's heat sink fans.

De-Schmutz the Keyboard
Keyboard's always catch dust, bread crumbs, and unidentifiable schmutz. To get rid of all this stuff you can take it outside and blast it with a can of air. An Avant Stellar keyboard which has four screws on the bottom can be cleaned by removing the screws, detaching the keyboard from the case, and using the air can there, too. You might want to try this if you have the courage--and the handyman skills.  Another handy tip is to use a tissue and rub alcohol to remove the grime that builds up on the keys.

Clean the Display:
Below are suggestions to keep your monitor clean

• To clean, gently wipe the LCD with a dry, lint-free soft cloth.
• If a stain remains on the LCD, moisten the cloth with water or a 50-50 mixture of isopropyl alcohol and clean water.
• Wipe the display with the moist cloth; do not let any liquid drip into the computer.
• Let the LCD dry before closing the lid.