Here is how it works in a nutshell. Every app you open appears in the app switching dock in the order that it was used, regardless of whether it supports a background task or not. This might sound pointless but the good thing is that it makes it much easier when you are trying to switch between a couple apps that you are currently working in, since they will be the first things in the dock. Although this list gets long (not sure exactly how long), it shouldn't matter because a) the recently used things are at the front and b) none of them are actually running in the background unless they support a specific background task. For apps with no background task, tapping on them in the app switcher dock is exactly the same thing as tapping on them on the home screen, and switching away from an app like this is exactly the same as returning to the home screen while using it- i.e. the app fully quits.
So the only type of app that you have to think twice about quitting yourself are the ones that actually do something in the background, and even those shouldn't be using any CPU time or RAM unless they are actually doing something. So keeping Pandora on the app switching dock doesn't matter unless you are actually listening to songs with it, and keeping Skype open uses essentially no resources unless a call comes in, so the only reason to remove skype from the switching dock is so that you don't receive any more calls.
