You may not need to install an AV but there are things you can do that will make your MAC and data more secure.
Basics:
Don't run your main user account for daily activities as an admin. Create a new admin account the remove admin permissions for your main account. You can still elevate permissions for certain activities when needed.
Use a firewall. I use Hands Off! Besides monitoring network activity it lets you sandbox applications from writing to disk if need be and allows you to stop specified applications from needlessly dialling home. Little Snitch is another popular alternative and OSX has a built in firewall but it requires some configuration.
Disable automatic login.
Advanced:
Use file encryption. OSX has FileVault baked in or use an alternative such as Knox.
Create a firmware update password:
http://support.apple.com/kb/ht1352
This stops someone form being to just insert a Linux Live CD and snoop around. Or to be able to wipe your drive and fresh install ie if your Mac is stolen. Which can make it easier to recover if you use an app like "prey project".
As for AV Sophos is probably the most popular for Mac and as said it can help stop potentially passing on virii to Windows users.