What Ozy said ...
The trouble with task killers is that when an app is sitting active in memory, it is not necessarily running and consuming CPU cycles. If you kill it and then launch it again later, it will need to completely reload the app, using more of the battery in the long run. And if you kill an app or service that is used by another app or service that *is* running, it will immediately respawn the process sucking up juice in the process. Auto task killing is the worst in this case as you could get into a power draining loop and your battery life will go down the tubes. If you think you have a misbehaving app, it's better to monitor it with something like
Watchdog Task Manager Lite - Android app on AppBrain
and either get rid of it and find an alternative, or, if it's an app you're stuck on, you can see where the drain lies and try to manage it or contact the developer.