Clean it when you get it to make sure there isn't anything in the barrel, if you want. Otherwise, if you were to quantify the difference between cleaning or not cleaning during break in, more damage is done to barrels by cleaning during the break in process than failing to "break in" a barrel.
If you clean it appropriately, no harm will really be done. There really isn't much to clean as it "breaks in".
When I bought my first two centerfire rifles, I read a ton on the internet and so I did the barrel break it. After those two rifles, I got connected with people and forums with good experience shooting. That helped me start sifting through all the BS that I was reading.
Many myths are hard to kill and are like zombies, coming back from the dead. It is good to question most gun lore and myths. Science has advanced and forums have helped connect more people to get information outside of the "gun writers" circles. Even today, many gun writers are still breaking through the old stuff.
When it comes to barrel break in, the myth remains strong. People expect to break in their barrel and for some reason the idea of not breaking in a barrel freaks some people out.
Yes, barrel manufacturers and rifle makers give "barrel break in" but when you talk to the owners of the companies, most will say it doesn't matter. But, so many people ask about barrel break in that they just list one on their website to meet expectations and avoid dealing with questions.
A lapped barrel definitely doesn't need to go through a break in procedure. The lapping takes care of nearly 100% of whatever barrel break in can manage to take care of.
If the barrel isn't lapped, you can lap it before shooting to clean up machine marks in the barrel some and make it nicer to clean and can improve shooting. But, the biggest spot that needs attention is the throat, but you can't really do much other than shoot it to break that in.
I know there is anecdotal evidence and people that have experience and believe in barrel break in. I've yet to see enough evidence that definitively supports the idea.
In the end, precision shooting is about confidence in yourself and your rifle system. Anything that helps your brain achieve a less stressed state of mind while shooting is worth it--if you have the money and don't do anything to damage it.
