I don't run AV software myself, but it's probably not a bad idea - it's sort of "the right thing to do", I guess. Honestly though, on a Mac, you really have no reason to, and I suppose if you forwarded PC viruses along to PC users this could cause some trouble, but it's not a big deal in the grand scheme of things. Myself, I'm on my Mac, behind a firewall, I install all Apple security updates as they are released, and my ISP employs excellent spam blocking and virus checking of all inbound e-mails, so there's no real reason for me to use AV software - I feel pretty safe. 😎