No you don't need one.

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Android Developers Blog: Multitasking the Android Way

There have been many posts about froyo not working with ATK. If all you want is to free up memory, the best way is to do it is by rooting your phone and installing an app call autokiller memory optimizer. Despite its name, it's not a task killer. It doesn't force close apps like ATK. Instead it configures android memory management to defined levels.

 

I havent used one since I uninstalled ATK a week after the store suggested it when I bought it. I use watchdog lite to keep track of cpu usage though, thats the real issue. Just to make sure apps arent going crazy using the cpu, thats where the battery drain comes in, not ram.