You're never safe, ever. Most people think of virtual machines as expendable because if it gets infested with malware, oh well, copy the clean backup over and voila! all better.

Not always the case. Core published a vulnerability in vmware just last month about how they could exploit the shared folders functionality in vmware to pop into the host system through the guest machine.

Even if you don't use that feature, the point is you never know what other flaws are waiting to be found or when they will hit you. The threat of attacking your host machine through the guest is already real, so you can't have the 'i'll just use a clean copy' state of mind, it'll come back to bite you in the ass...