My "server" computer, I leave on 24-7. This is my web server and email server. It is currently a P3 - 1GHz. It has been running continuously for 7 years now. The only time it shuts off is when we have a power failure. (No UPS on this). My other server is also on 24-7, but it is on a UPS.
Two of my laptops are on almost 24-7, and the third is turned on only when I use it. Otherwise it goes into standby. Both of my kids computers are turned on / off as needed. My main computer I put into standby or hibernation when not using. The family room computer is turned off / on as needed. My work computer is on 24-7.
I design computer motherboards for a living, and my larger team designs complete computer systems.
When you turn a computer on and off, the power cycling shortens the life of the hard drives. Leaving it on in low power state doesn't affect the electronics, assuming proper ventilation and cooling. Leaving it on *does* affect your power bill. It basically becomes a tradeoff between convenience, higher power bill, and replacement cost.
Oh, I also use a free online backup program to back up all of my critical data. Up to 2GB is completely free.
https://mozy.com/?code=QYWB4D
I've had way too many hard drives fail on me.
Disclaimer: If you sign up using the link (with the code) above, I get an additional 256MB free storage.
