The only exception you may make is if you are running the Classic Mac OS. Still, it's unlikely you'll encounter nVIR, SevenDust, Scores, or any of the other ancient Mac viruses these days unless you've got some really old infected disks laying around. A combination of Agax and Disinfectant will work; I don't install the protection INITs with them (Agax doesn't even have that option) and just use them as standalone products. Still, it's been years since I've seen a classic OS virus. Nonetheless, I do keep these utilities around just in case, as I do still get called to work on a very old machine now and then.
Any OS X system can go without anti-virus software.