...look...viruses can get on to your computer, but they will do nothing to it, unless it enters through windows based applications (like anything from the Microsoft Suite: Excel, Word, Entourage...)...
...i have Norton Anti-Virus just for shits and giggles...i run it like once every two months and usually catch nothing...its up to you...
...what I would suggest is got get your self a system optimization application (my favorite is Onyx, its free) nd run that every couple of months, too...that'll keep your mac happy, man...