I haven't found any difference in fouling between a copper jacketed or all copper bullet and I shoot both. But every new rifle I own had its barrel broken in using the shoot/clean/shoot/clean method. And every rifle I purchased used I lapped the barrel with J-B compound and also cleaned it good with a mild copper solvent like Hoppe's Elite. This can be a tedious process but pays off for the life of the barrel. And granted I've never shot a rifle enough to burn out a barrel like some guys have on here. Especially those who shoot competitively.
There are a lot of guys even in here that don't believe in breaking in a barrel. Well all I can say to that is "its your money". I've found that a properly broken in barrel will not foul near as much as one the wasn't broken in. I will also say the not all barrels need to be broken in. Most premium custom barrels have their bores lapped to a mirror finish which reduces the micro burrs that catch and hold the metal off the bullet.