I never heard of breaking in rifles before I came onto TFL in late 1998. I have two rifles I bought new in 1970 or 1971. The Wby MkV in '06 has some 4,000 rounds through it. The Sako Forester in .243 has some 2,000 rounds through it. I just took them out and shot them. Some of the first shooting of the MkV was to fireform some 100 rounds of brass, for reloading.
A year or so back, I sat down at the benchrest and got serious about holding for "best groups". The '06 put three shots into 1/2" center-to-center. Three shots from the .243 could be totally covered with a dime.
I use a copper-solvent bore-cleaner from time to time, if I've been a busy shooter. Otherwise, a patch with some sort of gun oil or WD40 through the bore. If you shoot, say, 40-50 rounds in a session, a good cleaning is worthwhile. If all you do is shoot three or four rounds to check your sight-in, and don't live in a damp climate, why bother? The bore isn't dirty enough to matter, and your only worry is rust.
I guess if my opponent was the guy at the adjacent benchrest, and we were trying for groups inside 0.1", center to center, I'd use every trick I'd ever heard of. Reliably very-small ten-shot groups don't come easy. If my opponent is Bambi, the gun is only one percent of the equation, and what I've always done with a rifle has always been better than good enough.
FWIW, Art