The process of lap fitting your prop to the prop shaft taper makes certain both the shaft and prop mate together on the taper well with no high or low spots and you achieve max shaft to prop mating.
When you see the inside of a prop with rings or high and low spots it was not reamed or lap fit properly. The mating of the two surfaces is actually very simple to do. They fit to each other through the act of rotating the prop around a locked shaft with the valve grinding compound in-between the two surfaces. The idea to to achieve a 70%+ contact area or better during this process. You can further check your work with Prussian Blue but it's not totally necessary.
Ideally, this should be done any time you replace a shaft or prop and are mating a new prop to an old shaft or if you are unsure whether it has ever been done. You DO NOT want to over do this though.
Step #2 Remove the key and apply a liberal amount of valve grinding compound to the shaft and slather the area where the prop will sit.