For the past 15 years I've always turned off my home PC at night, and I never turned off my work PC. Guess what: both were just fine. If you have a good reason for the PC to be up (downloads, server stuff, etc.) and the noise/lights don't keep anybody awake, it's fine to leave it on. If not, it's fine to turn it off.
If you leave it on and have a power-hungry gaming card you could also use nTune or Catalyst to underclock it when you're done gaming. That should save you a bit more in electricity bills. Also, you can select a power scheme that turns off idle hard drives after 20 minutes or so.