I started in with centerfire in 1950. My uncle never mentioned "break in" and he'd started with centerfire in the 1920s. I never heard of such a thing until here at TFL, sometime in 1999.
So, damfino. I've had plenty of sub-MOA groups through the decades, and never had any trouble with cleaning a barrel.
Now, a rough barrel can be cleaned up by burnishing, but that a whole 'nother deal.