I always run an Antivirus on my systems - nevermind if it's Windows, OSX or Ubuntu/Linux. Never think you are safe - a system can ALWAYS be hacked or infected - even Linux based distros. I remember the one time I got malware from Gnomelook.org on Ubuntu, because I installed a screensaver for my Ubuntu system. Read here.
And I have had 1-2 other Java malware coming on my Ubuntu system, because they were targeted for OSX and Windows, but it also ran on Ubuntu/Linux. Who knows what else I had during that time, because I didn't use an Antivirus.
Sure, the threat is much smaller right now (for Ubuntu/Linux based distros), but like Android: Once it gets more popular - you will see much more malware rise from the hidden ashes, because it's possible now already (my personal opinion combined with what I mentioned before).
I use the free and personal edition of "Bitdefender" on my Ubuntu system, because Avast wasn't working with "Ubuntu 12.04" and everything else was too difficult to set-up. Sure, ClamAV is good and easy (it's in the Ubuntu Software Centre), but their reaction time to threats (at least in the past) has been very, very slow. ClamAV also misses a lot of Windows viruses -> detection rate is not that good, at least when compared to commercial products (like Bitdefender, Avast, etc).
Verdict: Just my two cents on the subject. Never think you are safe, because you are not. A system can always be infected one way or the other - always. I haven't had a single problem with Bitdefender on Ubuntu and while an Antivirus/Security solution can never give you 100% security - it is better than nothing. I have been infected with malware both on OSX and Linux - on two systems where most ppl in the community said it couldn't happen or where I was told an Antivirus is not needed: Community Link
Oranges