On the topic of having a Task Manager app -vs- no task manager app on your Android, my view is I like to know whats normally running -vs- walking around in a daze not knowing. Just because you install an app like Advanced Task Killer does not mean you have to kill any tasks you see running.
I like the "Advanced Task Killer" app because it does not list all the extra system apps you should not be messing with. It allows you to Ignore a task you always want running like Gmail, and it allows you to view and manage (kill) other apps on demand.
In the "Advanced Task Killer" settings I've set the default click action to Popup Menu, and told it to Ignore Gmail, Google Voice, Email, and Advanced Task Killer.