I guess it depends on your situation. My thoughts are;

1. I prefer to load on a clean barrel because I leave the load in the gun until I shoot at game. That may not occur for many days, or weeks.

2. The first shot is the important shot. I want the gun sighted in for the clean barrel shot.

3. Anyway, a second or third shot on the fouled barrel isn't going to be that far off from a clean barrel sight in. And if you do your homework you're probably going to know where the second shot is going to hit in relation to a clean barrel sight in, and you can adjust your sight picture accordingly if necessary.

4. In almost 40 years of deer hunting with a muzzleloader I can count the number of times I've needed a second shot on one hand (with fingers left over).