FROM JACK E:You've reminded me, Mello, of what I read about the way Jaguars and I believe Mercedes used to treat the cast iron cylinder blocks of their car engines. After casting, they would dump them in a field, and leave them there for six months before machining them, and boring out the cylinders. What they found was that the castings changed in shape, as the internal crystal structure settled down, and the stresses caused by the rapid cooling of the cast iron sorted themselves out. Once thgat process was completed, they could be reasonably sure that when, for example, they bored out the cylinders, and the completed engine went into service, going through repeated cycles of heating up and cooling down, the cylinder bores would stay truly cylindrical, and their axes would all stay parallel to each other and square to the line of the crankshaft. If, in contrast, they machined the cylinder blocks as soon as they came out of the foundry, some of the blocks went slightly out of true in service - not enough to seize the engine, but enough to significantly reduce engine life.
In the case of air-cooled two stroke engines, it was well known by pro engine tuners and race mechanics that the running in process couldn't be done by just putting the engine on a test bed and running it continually for so many hours. It had to be done by repeated starting, warming it up, then running for a while before stopping it, letting it cool right down to ambient temperature, and then leaving it for a few hours. This temperature cycling had a dramatic effect of the extremely complex shape of the cylinder, with its ports running up through the cylinder walls, and allowed the structure to stabilise. If this repeated temperature cycling was not carried out, as soon as put it into racing conditions, the cylinder would change shape enough to seize the engine solid.
Mind, the crystal structure and crystal physics of objects made out of metal is an incredibly complex subject - that the London School of Science & Technology has a Professor of Crystal Physics says it all. This applies especially in the case of something with a shape as complex as a brass instrument, and is, as you point out, compounded by the myriad of processes which impose all sorts of strains on the finished structure. And think of the temperature changes the instrument suffers, when you take it out of your nice warm house into sub-zero air temperatures to play Christmas carols - and you blow air with a temperature close to 98 degrees F into the thin end of it, so that the temperature from the mouthpiece to the bell changes by about 100 degrees F!
Even though the brass used to make our instruments is a very malleable alloy of copper and zinc, I dread to think about the wracking strains imposed on them due to the hotter parts trying to expand more than the parts which are at far lower temperatures - especially on those outdoor Christmas gigs!