Leading can appear in two forms. Silver streaks in the grooves is the one most people think of, but it can also be a dull gray wash generally near the muzzle. That can also appear on the muzzle itself and shows lube failure. A coat of Liquid Alox on your remaining unloaded bullets should prevent that problem from recurring.
Either type can continue to build if not removed. The Chore Boy trick is as good a method as any.
Leading is mostly a problem for beginners and is the main cause of people giving up on cast bullets. It can be the result of bullets too hard, too soft, too small, too high a pressure, poor lube, or a rough bore. With experience, you learn what works and how to deal with problems, but it can be a rocky road when you are just starting.