"Do I leave my computer on 24/7 or do I shut it off when I am not using it?" We are here to say, "It really doesn't #@%&ing matter!" Ok, maybe it's not that cut and dry. If you need a concrete answer we would love to break it down for you.

To start with, set the monitor to shut off after 15 minutes when not in use. It'll save a few bucks, preserve monitor life, and will help prevent the screen from getting burned-in (unlike our site, that's bad!). The only time this does not apply is if the PC is being viewed for a lengthy period of time without user interaction, like watching a movie. The monitor is mentioned specifically because it SHOULD be shut down when not in use. But what about the PC as a whole? We're getting there.
If you share files 24/7, run your PC as a server, do folding@home / seti@home, or run any other program constantly, then leave the darn thing on.
The rest comes down to the almighty dollar vs. personal convenience. It may cost about $10 per month for the average user to leave their PC on 24/7. Some people like to leave the PC on every night to run tasks (anti-virus scan, defragmenter, etc). Others absolutely hate waiting for the computer to boot up. If either of these are worth $10 per month to you, then let it run 24/7.
If none-of-the-above apply to you, then turn your PC off when your not using it. Save that $100-$200 per year and go buy yourself something nice.
NOTE: Some people speak of concerns about hard drives having a shortened life due to being constantly turned off and on. The thing is, hard drive life expectancy is hard to judge; Not to mention, the replacement cost is typically well under the $100 you'd save in one year. In addition, the newer Solid State Drives (SSDs) are not affected by this issue. "BUT MY DATA IS PRECIOUS!" Exactly, that's what backups are for, but that's another tech tip...