JW, the definition of fouling is any foreign material in the bore. This includes primer or percussion cap residue. However neither 209 primers or percussion caps are corrosive. (at least not as fast as powder residue). The purpose of firing a couple caps or primers is not to foul the bore but to dry out any residual oil in the ignition area (breech).
Personally I never liked fouling the bore because I normally keep my muzzy loaded for the entire season. This includes my flinters. If I hunt in bad weather or suspect my powder may have gotten damp I will fire it off, clean the rifle and reload fresh.