I'm not an expert on this topic, but I recently did a lot of reading on the topic. You'll find 2 schools of thought:
1. "The break-in totally works, and will put you sub-MOA. I can tell a definite difference."
2. "The break-in is a ploy designed by the gun manufactures to wear out your gun faster. I did a break-in once and noticed nothing special."
You can read for hours, and that is about all the more you'll find. Some people try to make it more than that and dedicate whole webpages to the topic. I ended up deciding on not doing a clean-after-every-shot break-in after I came across an article written by Gale McMillan. Did I come to the right conclusion? I don't know, but for my purposes it probably won't make a huge difference.