Should unavailable buttons in a wizard be disabled or hidden?

On a wizard form I have the navigation, save, and cancel buttons.

Currently I disable the buttons according to the context. The save button is enabled when the user actually can save (ie, last page).

The QA Technician is telling me that the buttons should be hidden. I don't want to do that; because they are right aligned, the "back" and "next" buttons would be changing positions and I can't imagine that as a good thing.

So, should I actually hide those buttons? I've been reading about wizards but haven't found any place mentioning this particular situation.

Edit - Including sample of the page I mention.