preventing iMac from going to sleep while playing iTunes

When I'm using my iMac to play music through iTunes, I never want the computer to go to sleep. But when iTunes is not playing, I do want the iMac to go to sleep after a long period of inactivity. I don't want to change the energy saving settings to "never" when playing music, then change back with no music playing. I want to configure the settings once to accomplish this. Is there a way to do this?

I'm using the latest upgrade to Mountain Lion.

Steve