No, that is not true at all. The main reason for the small number of malware instances that affect Macs is simply the number of people using them. In the last few years, Macs only held about 6% of the market share. It simply sin't a big enough number to interest the malware designers. But, with the failure of Windows Vista, that number is now around 12%... and growing. This means that malware authors are starting to develop infections for Mac systems. Late last year, Apple announced that it now recommends to Mac users to install some kind of antivirus. As Chase said, better to be on the safe side, and having an antivirus installed is not going to hinder your performance anyway.