The people who shoot matches with really expensive percussion match rifles use Swiss brand black powder and usually RWS #11 or #10 caps , but they recommend using the lightest cap that will set the charge off reliably, and stick with that exact cap forever in that gun as switching caps can shift POI.
Will it matter for Joe Sixpack who hunts with a beat up Traditions Hawken percussion rifle and otherwise doesn't use muzzleloaders......but it matters to the guy shooting a Pedersoli Gibbs .40 who's in an NRA match shooting at 400 meters, where .1" can mean 1st or 35th place.