There are currently no functioning OSX viruses "in the wild," so at this moment in time the only threat is Word Macro Viruses (which CAN run on a Mac, at least sometimes--I've seen it happen) or possibly forwarding a virus-infected file from one PC user to another (someone sends you something infected, then you send it on without scanning it, that is).
I personally don't currently use any AV software on my Macs, and I also don't have any running on any of the Macs I administer at work--so far there have been zero issues with this (of course, at work the email is all prefiltered anyway, so that greatly reduces the risk of infection or passing something on).
I do, however, keep a close eye on Mac tech sites, so in the event a live Mac virus DOES get created (which is bound to happen eventually), I'll revise my policy quickly.
I'd say it never hurts to be cautious, except that's not true--the currently available AV software for the Mac that I'm aware of is better known for causing stability problems than fixing them. ClamAV is free, though, and while it's not as transparent as the commercial options, it also doesn't cause problems, so if you want to be careful that's what I'd recommend at this point.
