I've never had any problems with my Mac browsing around the internet. I have never used any anti-malicious software, but it has never been infected by any malicious software (to my knowledge at least). If I were you I really wouldn't bother.
There are very few known viruses for Mac and the majority of computers are Windows, so usually the malicious softwares are aimed towards Windows computers. Macs also have great security compared to Windows, so it's not worth the trouble for hackers to create these malicious files.
I don't think the data encryption on Yosemite changes a thing when it comes to picking up viruses on the web. The data encryption is more of a protection against physical acceess as well as a way to reduce the amount of data stored on your computer.
There are several good free anti-virus programs for Mac, though I can't give you any personal recommendations since I don't use them myself. Don't trust any anti-virus program though. Specific software for Mac have a bigger risk of being malicous software aimed for Mac users.
1Why do you think file encryption would stop you from becoming infected or attacked over the Internet? – Ƭᴇcʜιᴇ007 – 2014-11-27T16:50:34.963
PS: a new Macbook Pro should be able to handle running real-time AV and internet protection in the background without breaking a sweat. – Ƭᴇcʜιᴇ007 – 2014-11-27T17:07:32.480
@Techie007 you're probably right, but it always starts with a new computer, then the urge for protection, then performance decreases. So yeah, it is a concern. – Iancovici – 2014-11-27T17:30:47.233
If your Mac is infected, encryption means very little. If you (as the user) can see the data, so can the virus. HDD / Data encryption helps you IF you ever lose the Mac, and the thief want to get your data. For virus infection when you're using the Mac... Internet Security software may help (note: do NOT rely on such software and click on every link / run all software..) – Darius – 2014-11-28T02:21:58.970