In 22 years of Mac use, looking after a campus of ~200 Macs, ~900 users - I've never seen a virus that is a threat to the macs - but have seen mac users download or bring in windows viruses and have them stored in their home folders.
They were benign, but could be passed on to Windows users/computers so we do run ClamXAV on the OS X file-servers and delete these windows virus files which were mostly .exe downloads (sometimes in ZIPs) and malicious Office Macros.
If you do a lot of file exchanges with different Windows users you could install ClamXAV as a matter of politeness but I really would let them take responsibility for their own computers.