Never used any anti-virus apps in the 2 years I've been rooting my android phones. I've literally downloaded hundreds of zip files and flashed them onto my old moment and my current shift. Also I never use task killers.
Edit: Forgot to mention the effects on battery life, I cannot say for anti-virus on my phones but that is the reason I stopped using task killers, even if u kill the application it will keep trying to run in the background, same thing happens with the anti-virus apps. Plus the anti-virus app will pop up a notification asking you permission for everything, and scans every single last file you download. Correct me if I'm wrong just the experience I had bc i had to convince my gf family to take the anti-virus/task killers off their phones bc it was just wasting their battery life.