In my experience, it really depended in which apps I had installed. Some were buggy and constantly ran in the background when I wasn't using them, therefore draining my battery life. Others automatically closed when I stopped using them. Hitting the "back" instead of "home" when I was done with an app also helped.
In other words, I really never NEEDED a task killer, although I did have one installed on my phone in case an app was being stubborn.
I found it funny though, when I went into Verizon to get my Reality, I was talking about my Eris with the rep there, and he swore you needed a task killer or else it would run horrible. I showed him my Eris, how fast it ran, and I told him how good my battery life was, and it was fun to watch him think of what to say, because I wasn't running ANY task killer at the time. Lol.