I doubt it's the apps that were draining power. More than likely you had GPS, Wifi, or something else on in the background which was using up your juice. Most apps are doing nothing when they are in the background. Unix (which Android is built on) is very good at managing memory and processes. It's not like a Windows operating system where the application is constantly using resources as long as it is loaded. So the short answer is: You don't need to be constantly closing apps, because the operating system takes care of it for you.