Well, there's a lot of debate over the whole "task killer" thing. Some people praise the task killers, saying they wouldn't know what to do without them. Many others disregard them, saying they actually do more harm to your phone than good. In the end, they're both probably right, at least to some degree.
I haven't read too much about the truth behind the debate, but basically I've come to conclude that using task killers are perfectly fine if you gain a sense of satisfaction and ease from closing applications after use. On the other hand, if you're killing them because you think they need to be closed, then you're most likely wasting your time and slowing down your phone a little bit (because a few apps will just start back up automatically).