I've been using Macs since 1988 and always had some anti-virus software installed including, along the way, Norton, Virex and more recently Intego Virus Barrier. I only had anti-virus protection because I was operating a bureau service outputting Mac files from all the various graphics applications and was being hit with disks and files from literally hundreds, possibly thousands of sources over the years and wanted to make sure I neither received a virus or passed one back to one of my customers. In all that time I was in business I never saw a virus on any Mac disk/file I was given.
Nowadays, as I'm not using my Macs in a commercial environment, I don't have any anti-virus software installed that is constantly running in the background as they are resource hogs, I use the free (although they welcome a PayPal donation, but you don't have to) ClamXav which I just run periodically to check everything is still squeaky clean.
Sorry for the ramblings, but in short you don't need to bother but if you feel more comfortable with something ClamXav comes recommended by a lot of people on the various Apple forums.