The only time you need anti-virus software is if you regularly exchange documents and spreadsheets with Windows users. In that case, you need to scan the docs as you receive them or send them back, because you might inadvertently be forwarding a virus. The viruses will not do anything to the Mac, you simply want to avoid infecting Windows users.
Of course, the Windows users at the other end should notice an incoming virus, provided they're keeping their virus definitions up-to-date.