First off this linux, memory management is completely different. The processes that aren't in use and have been backed out of...return button instead of home...are in reality already "asleep". They take up a segment of your memory yes but with linux unlike windows there is little depreciation in performance, if any, when memory is in use and this is not what is causing much of the battery drain. The cpu being used is what causes the majority of battery drain. I do not use a task killer, but I do have systempanel just in case I need to control run away apps. I would refrain from using anything that automatically kills tasks unless your entirely know what you are doing and what you are killing. Many of the things that show up in your task killer will start back up after being killed automatically and all that you are doing is making the process active again which uses the cpu and drains battery.
I don't mean for this to be a discussion on using a task killer vs. not but if your goal is to mainly increase battery life try refraining from using a task killer, particularly one that auto kills, and see how your battery life differs. If there is no difference in battery usage this is healthier for your system because android itself allocates memory.