A match grade barrel that has been lapped and polished should not require a break-in ... but people do it to feel better that they gave the barrel every chance. Sometimes (rarely) those match barrels are still dogs.

Factory barrels may, just may benefit from a bit of special treatment. I've read somewhere (cant remember where) that the reamers used by factories are solid pilot and these can raise slight burrs and curls in the barrel forward of the chamber.

For what its worth, I'm in the clean them well, swap with metho to remove all cleaning product residues, I then give the bore a decent swap with a bore conditioner made by Tetra, patch it out ... then shoot the heck out of it! ;D
Cheers...
Con