Anti-virus in a Virtual Machine

VM guest OSes can have security issues that impact the host. VMware regularly provides patches related to this and I imagine that other VM providers have similar issues.

I do not believe a virus on a VM can affect the host OS, but the VM itself can certainly be infected. I don't run anything other than MS security essentials on my Win7 VM, but I don't use it to do anything other than a couple of applications- certainly no email or web browsing.
I do have a couple of XP images that I keep backups of, so I can boot up a clean machine to test anything, and roll it back in seconds.

There are a lot of variables here that you don't clarify (such as VM OS, is this just a development or test VM that you're willing to wipe at the first sign of trouble), but yes it's best practice to run anti-virus software in your VM. Are you concerned about cost? There are lots of free alternatives out there. The question is, why would you not want to run anti-virus software?

If you are talking windows, which I'm presuming you are... yes, you should have an AV package in the VM. If nothing else, it makes windows happy. Get the free but effective Micro$oft Security Essentials from microsoft dot com.
I run two VMs, W7 and XP in Parallels on my Mac, both running MSE.

do you surf the internet with the VM, or check email, or run programs from unknown sources?

yes to any of the above, then yes the VM needs its own antivirus. as far as the VM knows, it's all by itself, and as far as the host (parent) knows, the vm is a "black box", it can't really see what's going on in there.