Always check the plug fit BEFORE you inlet the barrel. On some barrels the plug face is 1/16" shy of the bore. This is not unsafe, in my opinion, but makes the barrel a chore to clean. If you don't know there is a fouling trap at the plug, likely you will have corrosion begin in short order.
If you do need to fix a gap at the plug, best to do it before you have the barrel inlet. The barrel may end up at a different flat by the time you get the plug seated properly.
While the plug is out, it's a good time to run a patched ball down the bore to make sure it doesn't have loose spots at the muzzle, rough machining marks, etc. If find bad spots, and you haven't made any cuts on the barrel, it's easier to return it to the manufacturer.