It has nothing to do with whether you're rooted or not, the last OS that even needed a task killer was FroYo, since then the OS is quite capable of managing memory effectively, task killers on ICS use more battery and do absolutely nothing as far as speeding up the device. ICS dynamically tracks memory and only kills processes when necessary thereby keeping apps in memory for quick launching and the battery savings by not constantly having to load the same application every time you use it. If you don't believe me just do a quick Google search and I guarantee that probably the whole first page of hits will agree.