An out-of-the-blue virus won't be caught by your AV software anyway, so what's the point. AV software can only scan for known virii, and therefore can only protect you against known virii, and since there aren't any, there's no real need for AV software. If you're worried about Office macro virii, then just switch off macros, and if you're really paranoid, just use ClamAV... at least it's free.
Top 8 OS X Safety Tips