Not buying into any argument, Apple instruction manuals advise shutting it down when you are going to be away longer than a few days. Here it is:-
Putting Your iMac to Sleep or Shutting It Down When you have finished working with your iMac, you can put your computer to sleep or shut it down.
Put Your iMac to Sleep: if you will be away from your iMac for less than a few days, put it to sleep. When youriMac is in sleep, its screen is dark and a small white light below the lower-right cornerof the display pulses. You can quickly wake your iMac and bypass the startup process.To put your iMac to sleep quickly, do one of the following:
 Choose Apple ( ) > Sleep. Press the power (®) button on the back of the iMac. Choose Apple ( ) > System Preferences, click Energy Saver, and set a sleep timer. If you have an Apple Remote, press and hold the Play/Pause (') button for3 second