I meant learning how to keep them all working over time. I read several places that said that changes to "master" shapes may not be propagated to the drawings because they get replicated in document stencils, or something like that. So it seemed like there were some pitfalls, which I would almost certainly fall into. So I wanted to understand the proper way to create, use, and maintain (manage) them all.
I think there is not much more to it, actually, than what you just said...
It is just to understand that Visio files are self-contained. i.e. everything a Visio file needs is contained in that file, including masters.
Therefore, if you modify a stencil file (i.e. a different file) this change will not actually affect things that are outside of that file, i.e. in all other files, the master (and shapes produced from it) will stay like they were before the modification.
This is all only relevant if you build your own masters and stencils, of course. Most users probably don't and just use the existing stencils provided by Visio.
I remember long, long ago, Visio even came with some database scripts to store custom masters, and some tools to manage them
But that was all much too complex, and was probably dropped at some point of time in the past.
