Hmm. This is something I've been struggling with myself. I've never heard the angle that you SHOULDN'T use a task manager/task killer. I have a habit of constantly checking my running apps and freaking out if I done have at least 150 mb or so of free ram. I must kill apps running in background a dozen times a day. But to be fair, it all started with battery drain problems. Some days, at random it would seem, my battery would drain MUCH faster than usual, and upon checking what had been using it,I found Android System at the top, every time, with 70% or more. In fact it uses twice as much battery as the next thing below it on these days. And as far as I can tell, all it is doing is holding the wallpaper.
Maps is also pretty bad at constantly running, and it drains a fair chunk by itself, even though I almost never open it. So I quarantined that. So you're saying, just because apps may be using ram in the background, they aren't necessarily using up battery?