Since tkg is not here to answer, I will pass along a tip I learned from him:

Paint the shaft taper with a permanent marker, then install the prob hub (without the woodruff key if it came with one) and turn the hub around the shaft a few times. When you remove the prop hub, if the permanent marker paint has been worn off very evenly, then you are probably okay. However, if it wore unevenly, or only the front of rear portion of the taper is worn off, don't trust it. Even if you are a little bit off, the prop will wobble around and cause mayhem.