The mac OS does a very good job of caring for itself. Few if any AV products will allow you Mac to run at its full potential. If you avoid suspicious download hosting sites and Torrent sites, you greatly reduce your risk of getting something messy. Any site offering files free that you would normally have to pay for will almost always install adware.
The only real threats--more of a nuisance--to Macs is adware. Most AV products will not stop those. Sane surfing practices work best. For additional information on adweaare and how to avoid them, pleasse see this series of articles:
http://www.thesafemac.com/tag/adware/