Don't believe anyone who says that Macs are 100% secure and malware free. There have been several notable instances where after hearing this many people have found themselves in the midst of a Mac malware outbreak (MacDefender, DNSChanger, and Flashback, just to name a few).
These attacks have been rare, and with "many people" being around 1 percent of all Mac users, for now this has been far of an issue than on other platforms, but security issues do exist for Macs.
Whether or not this warrants the use of special security software is up to you.
My recommendation is to keep a low-key and free malware scanner on your system, such as iAntivirus, ClamXav, or Sophos Home edition that can be configured to only run on-demand, and then should you feel the need use it to run a scan of your system every now and then. You can also use a reverse firewall like Little Snitch to monitor all outbound connections, which was key for many Mac users who found evidence of the Flashback malware on their systems when that outbreak occurred (just see here for the thread where many detected the suspect behavior: https://discussions.apple.com/thread/3844172).
Since malware is still quite rare on Mac systems, it is debatable as to whether or not security software is necessary, but my stance is if configured as I described then there is very minimal chance it will adversely affect the system. I've configured numerous Macs with anti-malware utilities and have not seen any problems with them, so for the most part there should be neglegable impact (if any). Keep in mind some security tools are more intrusive than others, and include a number of scanning routines that are enabled by default, so each one has its own approaches that will need to be considered. The ones I mentioned here are very light-weight.
The thing that will keep your system safest is (besides "common sense" safe computing practices) to use plugin managers and ad-blockers to prevent Java, Flash, and other plug-ins and scripts from launching automatically.
