1. I'm not entirely sure AV is necessary, though it may be in the future. I don't personally run one, but I wouldn't tell anybody not to if it's what they want. Like SyntheticHuman says just above, if you're careful (ie, don't go installing warez from Chinese websites), it's probably not needed.
2. Task killer. If you're reading or hearing that you need one, that's holdover legacy thinking from pre-2.1 Android. Android actually does a good job managing apps/memory on its own, and task killers can (and often will) lead to problems. Trust the OS. If you're running Android 1.5, maybe... but Froyo, Gingerbread, or later? Unnecessary and counterproductive.
3. Battery optimizer... I've gotten far better results rooting than I ever did with any battery optimizer. I've got a total of five android devices that I take care of (self and family, I'm not a dev or a tech guy), and every phone that I've rooted and flashed a custom ROM to has seen improvements in battery life (and usually improved stability, not to mention added features.) My dad's Optimus S gets about 50% more use between charges since we flashed a new ROM.
As long as you're willing to do a little reading, and can follow instructions, I'd absolutely recommend rooting and trying a custom ROM. For the LG Optimus S, I think you're in the best place--there's lots of forums out there, but this site seems to have the best info and the most current discussion for this device. Read up, ask questions where you need clarification, expect some trial and error... but you'll be surprised what this little phone can do when it's tweaked properly.