My best guess as to percentages.
Those on this forum with over 500 posts - 90-95% do not use an a-v app.
Those who have been using OS X for 3 years or longer - most likely to recommend you do not put one of those apps in OS X.
Those on this forum with under 100 posts - 50% of those that post in these threads will recommend you use an a-v app.
Those who have been using OS X for less than 1 year - most likely to recommend an a-v app.
Those who have been using OS X for 5-10 years or longer - most likely to cower in fear at the mention of putting an a-v app on their machine.
Don't know that we have anyone here in this class that uses an a-v.
I am in the first group and definitely recommend you do not install an a-v app in OS X until such time as there are any virii in the wild. If you install Windows on your Mac, then you'll need an a-v program there same as any other Windows machine.
Percentage of users reporting issues with their a-v program vs users reporting finding virii/trojans/spyware/malware on their Mac in OS X - about 9:1.
The first post in the following thread is just as valid today as the day it was written and provides you with all the info you need to make a decision as to whether you want to use one or not.
http://www.mac-forums.com/forums/switcher-hangout/23835-official-mac-antivirus-firewall-faq.html