I generally use virtual machines for testing, however, you can sometimes run into problems with this. For example: I recently discovered a javascript / flash interaction bug in IE that only exists in a VM environment. In the past, I've run into strange IE css rendering bugs in a VM as well. These are really few and far between, but they do happen, and have been documented issues with virtualized environments. If you are testing on a VM and you run into a bug that just doesn't make any sense, try running it on a real box with the exact same setup.