I'd have to strongly disagree.
The backend of macs is now a Unix type of affair. Hackers have long looked at exploits for these kind of systems. A firewall (even the built in one) would be a must. If you're connecting from home, most routers supplied by broadband provides pretty much nail their routers down to only accept traffic that had an originating request from the internal side of the router.
As for viruses - agreed that there are hardly any that affect the Mac - most are proof of concept and don't exist 'in the wild'. However, it's long been accepted that the Macintosh platform didn't have sufficient numbers of users for hackers to target (for kudos) or exploit (key loggers, trojans etc).
As someone that uses a Mac but also worked as a sysadmin during the rise of Viruses like melissa, 'I love you', MyDoom, etc. I've seen how a proof of concept virus can infect literally millions of machines in a sub 24 hour period.
Eventually, a virus or a program exhibiting virus like behaviour will appear and there's a lovely, big community of users that haven't done anything to secure their machine..
...also, you''ll have a Windows machine running on your Mac - that will definitely need virus protection.