Seems this question comes up from time to time and I think the best answer is to follow the recommendations of your barrel maker. I wouldn't bother breaking in a factory barrel, but the few rounds it takes to break in a custom barrel I believe is worthwhile.
The break in has nothing to do with the bore itself, which from the good barrel makers is glass smooth. However, when the chamber is cut the reamer leaves microscopic ridges across the grain of the steel. Once these ridges have been built up with bullet jacket material, they are coated and protected. The heat from subsequent firings hardens them so they can never be removed. Every time a round is fired from then on, bullet jacket material is stripped off and suspended in the propellant gases and is subsequently deposited along the bore, which we recognize as fouling. Once those ridges are heat treated, that barrel will always foul, and nothing will resolve that problem. When the round fires, it is not the passage of the bullet that removes these ridges, it is the heat and pressure of the propellant gasses, as the bullet is already gone.
My Krieger cleans up so quickly that I am convinced the procedure is worthwhile.