The best reason for using shim stock is that it is very thin, and you want the edges of the pinhole to have the smallest possible thickness. The classic technique of using a blunt punch to make a dimple, and then sanding off the convex side until the pinhole is the desired diameter, will give you a thinner edge than any shim stock that is robust enough to handle. Heavier starting stock, within reason (soda can and on up) is actually easier to work with.