There are different types of lockwashers. Some are split ring. Some are star washers. Those introduce uneven stress into the prop hubs. That is something that, when allowed, is very controlled. For example, drive washers often have shallow radial serrations, but drive washer tolerances are tightly controlled by the manufacturer. And those same manufacturers have chosen over the years to use lock nuts whenever they wished to strengthen that area. Look at 4strokes. Almost all of them come with a nut and lock nut.

You could use a lock washer between the prop washer and the prop nut, but a lock washer has little effect on the hardness of the washers used and the nuts used. And to use one against the front of a prop would not be a good idea.