back before android 2.1 ... Task Killers might have been useful..
android is not like other OS that you have been used too... the old way of worrying about what is running in the background and using up RAM is so 2010. leave the past.. embrace the now.. drop the TaskKillers and forget what is running in the background.
android takes care of it for you.. and wants to use up as much of the RAM it can.. by using taskkillers you are messing with android basic functions.. and it may impact your performance, functions, and battery life.
there is a built in task manager.. to help you see what is back there and if you want to kill them off.. but again: dont even worry about it .. till you feel there is a mis-behaving app.