It has been my experience that steam locomotives do not need extensive "breaking in" after a rebuild if the work is done correctly. It is certainly a good idea to run lite for a short period of time which is determined by what work was done. Historically the "real" railroads' quality control was less than ideal. There was a tolerance for every repair operation, and that was usually not too close. Having worked on a locomotive that had never been run after a major rebuild by the Pennsy, I was amazed (appalled) at the quality of workmanship. It looked good, but needed a lot of fussing to get it to work properly.
I have seen locomotives rebuilt today that go into service immediately with almost no break-in period and perform flawlessly. This is almost always due to the care taken in the rebuild. There are quite a few very capable steam people in our industry whose standards far exceed the historical standards. You only have to look at the long term track records of those folks to see who knows their stuff.
That being said, steam locomotives are steam locomotives and they are full of surprises and are capable of serving up humble pie at any moment. Additionally, the best rebuild in the world will not survive operational abuse for very long.
I guess my point is that given reasonable care in operation, I do not buy into the myth of needing a long break-in period for a correctly repaired or rebuilt locomotive. Our experience at Strasburg has been that if a problem arises and persists for any length of time after a rebuild, it is usually because we did something wrong, and it must be fixed. The problem rarely goes away by itself.
I am mainly speaking about running gear. Boilers are a different animal - especially riveted boilers. From a historical standpoint, based upon reading I have done and stories from the Pennsy boilermaker under whom I served my "apprenticeship", new and repaired boilers were prone to leak. I read a description of a new Baldwin that used the word "rainstorm" to describe the hydro test. Structural defects aside, riveted boilers need their pal rust or some welding to become completely water tight, especially considering the limited or non-availability of steel that is as malleable as what was used "back in the day".
In the end it comes down to "pay now or pay later - with interest". Steam locomotives will make you pay the full fare in the long run.
I mean this only as a general observation on steam locomotive rebuilding to be filed in the "for what it is worth" department - or in the circular file as one may choose.