Don’t use task killers with Android 2.2 and higher.
Android in essence is prepared to handle inactive tasks all by itself. What does this mean? Well, in essence you will have two types of apps running in the background: apps that remain silent in there but are active (certain widgets, and programs that are loaded with a feature to start up faster… think of adobe in the PC), and others that basically are programs that you use and you exit or simply minimize to the background by hitting Home. The resident programs really consume very few resources and the ones that are minimized will be shut down by Android after a certain time of inactivity.
Moreover, if you have ever used a task killer like Advanced Task Killer, you will notice that by hitting the “Kill All” button, well, the app will do just that. However, if you go back into the app 3 seconds later, you will see that most of the apps that you were trying to kill, will be back up and running. The point here is that the system will restart every single one of the apps that you tried to kill and may end up causing system instability, leading you to Force Closes, or even lock ups.