The main reason you break in a new barrel is to make it easier to clean. If there is any improvement in accuracy, it's nearly impossible to measure. That said, if you are talking about hunting rifles that get cleaned a couple of times a year, it's not worth it to break them in.
On the other hand, if you have varmint rifles that might get cleaned ten times in an afternoon's shooting, it is worth doing. You can break in any barrel at any time as you are just smoothing out the tiny rough spots in the barrel by shooting thru a clean barrel. You have to clean thoroughly between shots as the copper fouling will protect the tiny burrs you're trying to remove. You'll know when it is broken in as the barrel will clean much easier. Good luck