The answer is on page 16 of your owners manual where it says:
Putting Your iMac to Sleep or Shutting It Down
When you finish working with your iMac, you can put it to sleep or shut it down.
Putting Your iMac to Sleep
If you‘ll be away from your iMac for less than a few days, put it to sleep. When your iMac is in sleep, its screen is dark. You can quickly wake your iMac and bypass the startup process.
Personally my 4 year old 2007 iMac is left on 24/7 and I have the display set to sleep after 15 minutes of non use. The machine is a stable today as it was when I got it.