I sight in with a clean barrel...or should I say I never shoot more than two shots before swabbing when actually sighting. However, after sighting in, I intentionally shoot a bunch of shots...generally 5 or 6, without doing any swabbing of any kind to see how the fouling may affect the shots at various ranges. Have never needed more than one shot so far, but who knows what might happen and I want that info tucked away in my head if needed.
BTW...on my guns and loads I've found the difference clean or dirty is within "Minute of Deer" accuracy anyway. That's good, 'cause my memory ain't what it used to be!