You will get many ideas regarding the "need" for antivirus and malware protection. Many people here are seasoned users who recommend against it, largely based on what they consider easy steps to either avoid or detect malware presence; however, (and in no offence to them) these steps are not always obvious to everyone, so it's largely a subjective matter.
I recommend you give Thomas Reed's Malware Guide a read: http://www.thesafemac.com/mmg/
This will give you a pretty good rundown on the details of the Mac malware scene and considerations for security software.
Overall, however, a lightewight and free anti-malware utility will not hurt your system and can be set to only scan periodically to help you detect any known malware with minimal impact (if any) on your system. Unfortunately malware does happen, and while it is still rare in OS X and while does have a number of built-in options to help prevent it, the possibility is there and security software is simply another tool to help manage it, should you encounter it.
Some such tools are Sophos Home Edition, and ClamXav. Others will work as well, but you may have to limit some of their default behaviors as they may try to activate full-heuristic scanning routines that can sometimes be a touch intrusive.