In that case, it's better to have antivirus software on the PC ... you're far more likely to get a virus from some other source than from some file on a Mac.
Also, again the logic here is that files entering a Windows PC from a Mac are like files entering a Windows PC from anywhere else. Either the PC will successfully detect and stop them or it won't. If your ecosystem is bringing in PC viruses that PC AV cannot stop, then you aren't solving this problem just by trying to find them on your Macs (which probably also won't be able to stop them), because a user on a PC could just as easily bring the same virus in doing the same thing you're doing. There's no differential safety in doing AV for PC viruses on your Mac.
Not to mention that really, the only way for transmission to begin with is if you download an infected file and then give it to a PC user. Your Word documents or the PDFs or whatever you generate can't get viruses on them because none of the viruses replicate on a Mac and they're newly created files.
I keep ClamXAV (anti-virus) on my computer because it's free and it's always there if I need it. But I rarely use it. It's not bad, though, and it's frequently updated, and if it gives you peace of mind, by all means, use it. It's good at least in terms of principled support for the OSS community.
I keep MacScan (supposed anti-spyware) on my computer because I installed it on a whim when we were curious to see what it might detect, and I only keep it on my computer because I'm too lazy to get rid of its worthless carcass.
