Shutdown

There are dozens of threads on this, and what you'll find if you read them is that it doesn't matter. You can leave it running without shutting down for months at a time with no ill effects.

Is it 'bad' to never shut an iMac down, but instead sleep it?

 

I keep my iMac on 24/7. I even left it on during a 10 week trip this past spring. I continuously synchronize all the data I use on my MBA through dropbox back to my iMac. From there it is double backed up in the house, and to the cloud. This keeps all my data safe even when traveling.

/Jim

 

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.

 

I leave my iMac on 24/7, I actually have it set to go to sleep after 2 hours. I did the same with my 2007 Macbook Pro when it was my main computer. You should be fine with leaving it running 24/7

 

Either way works as people above have stated. Due to the way that metal contracts and expands when heated and cooled, you could possibly get 'chip creep' (chips dislodging from where they are soldered) by turning it on and off but thats an almost none existent possibility in modern systems.