Back in my 486 days and up until my pentuim 3 days I used to keep the computer on and just turn off the monitor. It's the monitor that uses the most electricity of any component, so shutting that off saves your electric bill.
Now-a-days with CPU's putting out enough heat to cook on, I'm too afraid of fan failure to leave the system on at all times. The most I'll leave it on for is a day.
The theory is that leaving it on will prolong the life of your computer due to less "power shock". Thay may have been true in the olden days, but today's computers put out enough heat to cook themselves to death. Besides, who uses a computer more than 2 or 3 years old anyway? This isn't a TV or a fridge that you expect to last 20 years. You expect a computer to last 2 years, maybe 5 at the outside. Power shock won't kill it in that amount of time.
Of course, if you're talking about a server then ignore the above.
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