Actually, the way the OS is written and how it handles applications means when they are not in use, even when in the "background" they use pretty much zero memory. Its extremely efficient. Unlike BB's ect where they cause the OS to get sluggish, you really wont see this on Android.
You really shouldn't need one for anything other than just in case you get a bug, you can shut them down. So for that its good to have around, but don't feel like you need to close things just because they are "running in the background", because that means something totally different on this OS.