Shortly after buying my first Android device I became obsessed with understanding how multitasking in Android works, and I mean how it Really works, not just "press the home button and X will happen.".
Only one person, across several so-called expert forums, ever gave the real story. Since then I verified that story by reading the information given to Android application developers on the official Android development site. Here is the summary.
Android is designed to take advantage of caching. It is designed to use memory to keep recently and frequently accessed content readily available. It is designed to remember what was going on - taking an instant "snapshot" of where the app was - so it can return to it quickly and efficiently (read: return to it and context switch using as little work, so as little battery, as possible). This also means, for the most part, free memory on an Android device is effectively wasted memory.
So, assuming the app is properly designed, the moment you place it in the background by switching to something else it should be consuming Zero resources unless it actually has work to do (most do not).
So, what really happens to the average app and to your phone when you download "Task Killer 123" to make sure "those darn apps close for real!", well first of all you have another 24/7 active app running, which eats more resources. Then when you "kill" stuff, you lose your place in the app for one thing (it's pretty sweet to be able to switch away from a game in the middle of a level and instantly be back where you were, two weeks later). Second, you exert resources it shutting down, and even more resources starting it again later.
To conclude, this dude was the only person explaining what the design intent actually was, which nobody else seemed to understand: "You don't have to worry about Exiting stuff. Just jump to the home screen and go do something else. Unless you are running like 50 apps you should not be 'running out' of memory".
So yeah, please stop assuming you need cleaners, advanced task managers, tweakers, rooting, and all sorts of other stuff so you can delude yourself into thinking you are "improving your phones performance.".