Where do I fall on this spectrum?
As a wise man once said, 'a stitch in time saves nine.' Sure, there are no known viruses for OS X in the wild (there are some 'proof of concept' viruses out there) but that doesn't mean there will never be a virus for OS X.
But Scott, why would someone bother writing a virus for OS X when Windows has 14000% marketshare? Many people point to this fact and sleep soundly because of this concept of 'security through obscurity,' but thanks to the iPod the Mac is on everyone's mind.
You have to ask yourself, is my computer that important to me? If it isn't, well then you have no need for antivirus software, enabling the firewall, or using strong passwords. However, if you store sensitive data on your machine, or you couldn't think about a day without your Mac you really should get yourself some antivirus software and you don't even have to pay for it.