You can just put it in sleep mode for the sake of energy savings however it's actually better to leave it on because the maintenance scripts are performed in the middle of the night. The scripts clear out unused ram in the inactive state as well as stopping unnecessary services in the background. So for the sake of self maintenance keep it running constantly.
I never shut my iMac down and it works just fine. The only time I ever reboot is when an apple update requires it. However, from a green standpoint this is probably not the best for mother earth (or your wallet).
I put my imac on sleep everyday. The only time i shut it off is when i'm not going to use it for a couple of days... That's what the manual says to do anyway.
My macBook i also put it to sleep because i usually end up using it more than once a day; everyday.
So my imac gets shutt off like once every 2 weeks or 3