I personally don't kill any apps nor do I use those task killing apps. As far as battery goes try adding the Power bar widget to one of your screens and turning off wifi, gps and changing the brightness when not needed. Works for me.
Never needed a task killer, The android OS has built in task management. I suppose if you like micro managing the apps running, then OK, but I think it is a waste of time for the most part, because in settings, you can clearly see if something is consuming the battery. If that is the case kill the app for ever.