I was told originally from the Apple store where I bought the MacAir that I did not need to run any additional antivirus on the mac.
You were told correctly. These aftermarket so-called "Virus Protection" packages make a BIG SHOW of scanning files all over the place for supposed "Threats" to your Mac. They may give you a feeling of protection. They do nothing worthwhile. Everything needed to protect your Mac is in one of these two categories:
1. It shipped in the box with EVERY Mac, not just the ones from Best Buy.
2. Your constant vigilance and skepticism are required. Almost every threat today requires your complicity. Be suspicious, they ARE out to get (not YOU, your credit card and bank account numbers).