There isnt really any need for a task killer for Android. Running apps are ok as long as they arent hanging up on you (errors, freezing) Android is a good self manager of its memory resources so its not needed to kill apps.
Plus, if you do install a task killer and kill off apps, you will notice that apps will open themselves up in the background, thats what Android does, and it would kill your battery constantly killing these apps off, since they will come back. Dont worry though, thats just how it works, and isnt a bad thing at all.
I have a task killer installed, but not to auto kill anything, the only time I use it is when an app is hanging up on me and it must be killed on the spot. But thats not too often.