I have left task killers behind. haven't missed it,
have read that the phone handles this well itself, and will actually turn stuff back on if it thinks it needs it for something. Been advised numerous times to dispense with it.
better for battery life are things liek Juice defender and setcpu with kernels that support it, using a screen off profile that under clocks the phone.
and when on an aosp rom, using a kernel with sbc that allows the battery to get a full charge in the first place.