Two part answer. Yes just use your back button when you leave an app this is better then just pressing home. Second, when you're on you computer you see several sites you have opened across the top and on a computer all are running in the background and this is what we are accustomed to. Now on a android phone when a app is shown in the running services its actually froze there not using your processor. This is how it was explained to me...if you were at home and wanted to play 3 board games and they are all in the basement do you run up and down the stairs 3 times or do you bring them all up at the same time? When their all there you can use one but you have the others right there when you need them. Your phone well clear out the apps it has not used in a while as it see's fit
This is why app killers are harmful to use. Some apps need to run for your phone to run correctly like weather and such. I know this is a long winded answer but i hope it helps. If you need any more help just ask
So I am using advanced task killer. I have it in crazy kill level. I should not be killing everything in the manager?