I've never really closed apps as a regular thing either, maybe once every two weeks or so when I'm just messing around I might. And back when I had a 3GS and 4 I'd close them somewhat regularly. But now there really is no need at all. iOS does an EXCELLENT job of balancing out RAM usage so that it doesn't prevent any other apps from not opening. It just auto releases the RAM used up by open apps that haven't been used in awhile.