Honestly, unless you know EXACTLY(HEAVY EMPHASIS) what an app does and EXACTLY(HEAVY EMPHASIS, AGAIN) what it affects - whether its basic functions of the phone to other apps that may be connected with it - you should NOT use a task killer. It will give you more problems than help.
Android is very efficient in handling its own task killing and just because an app is opened in the background does NOT mean its using processing power as it can be throttled to essentially nothing. In fact, most of the time killing an app and having it start up again use MORE processing power than having it sit idle. Unless the app being questioned is constantly sending data packets/syncing, it will not affect your battery life. As for the RAM issue, again, Android is very efficient in handling this. It does a terrific job trafficing RAM to where its needed, and only where its needed.