Lookout drained my battery and slowed down every phone I've had it on, so it never lasted long before I would disable it. Most anti virus/malware removers don't do much of anything, really. Most only do something after the fact your phone has been 'infected' with something. Only install apps from the play store, be careful with what websites you go to and don't click on strange links in emails and off texts and 99% of the time you'll be fine. The same precautions you would use on your computer really.
Honestly, the bigger risk is loosing your phone or having it stolen and then not having the ability to remote lock or remote wipe it.
If you'd still feel better having one, I'd recommend downloading something like Lookout once a week or every other week and running a scan and then deleting it. It's just going to hog system resources if you leave it on 24/7 and most of the time if won't stop anything, just tell you something has happened after the fact.