Task killers are not needed but again needed for poorly coded apps that do not close all the way when hitting escape key.
But your better off just letting it run and once you phone starts to lag, just do a reboot.
It's kinda like your Windows computer, when some apps don't close all the way and are in the Task Manager, you don't go and kill it, so why do it to your device. Yes Task manager is there but you rarely use it and when your computer starts to run slow, what do you do? reboot.