[QUOTE=Leaper]
I know I’ve read that there are pluses and minuses to each, but I’ve never seen anything definitive on which side tends to be greater.
[/QUOTE]
You aren’t going to find anything definitive, because it varies too much.
If you leave your computer on all the time, friction and heat try to kill it. Moving parts wear out, and electronics deteriorate over time. You are also succeptible to power line spikes and sags which can damage things.
If you let your computer sleep, you will alleviate some of the friction and heat problems, but you are still vulnerable to power line spikes and sags. You also will have “thermal cycling” where different parts of the computer will be heating up and cooling down. Dissimilar materials expand and contract at different rates, which puts stress on the parts as the temperature goes up and down. Solder joints slowly fail (especially if it wasn’t a very good solder joint to start with, which sometimes happens during manufacturing) and the little metal wires inside the chips which connect the silicon inside to the pins on the outside tend to seperate from the silicon after awhile, rendering the chip useless.
If you turn your computer completely off and on, you also don’t have so much friction and heat problems, and you are much less likely to have power related problems. The thermal cycling will be even worse, though. There’s also the “surge” that chappachula mentioned.
Which of these effects is greatest depends on your computer and everything around it. If you live in an area where power spikes are common, you’ll be better turning the thing off, but if you turn it off and on a lot (like every hour or so) you’ll really kill the thing due to thermal stresses. If your computer runs a little too hot (as many computers do these days, especially laptops) then leavnig it on all the time is the worst thing you can do. Excessive thermal cycling with something that gets really hot is worse too. Heat is a big killer in computers. If the room you are in is not air conditioned, it may be better to turn the computer off when you are not using it in the summer, but leave it on in the winter.
It’s generally better for monitors to have them shut down when not in use.
By the way, even if you never ever turn your computer on, there’s an aging effect which goes on in silicon which will try to kill it. No matter what you do, there’s something trying to kill your computer.