As long as your number 1 rule is common sense then you have NO need for antivirus/antimalware on the Mac OS. Of course, I guess the same could be said of Windows, but it is even more true of Mac.
EDIT: IF you share files with a Windows computer, an on-demand virus scanner is handy, just to prevent copying a file that might infect your Windows computer.
If you follow tech news (probably since you're using this forum?) you'll know about any real Mac malware threat which actually comes along. Through the entire history of OS X you pretty much had to be doing some pretty reckless things to have encountered malware and let it cause trouble. I guess the only advice I would have for someone is to be a little extra cautious if they pirate software.
A passive anti-virus scanner (there are free options) is nice if you want to scan documents before sending them over to Windows. Unless you had a good reason, I wouldn't infect your Mac with a 'real' anti-virus program (like Norton)the slowdowns and problems just aren't worth it.