Programming

Does this need to change in ALL places? How do you know? How can you get all places it is used and be certain it has to change in all of them?

These seem like questions that are equally important and hard to answer regardless of whether you're using abstractions, although abstractions have a big advantage in that you can find all the sites where they're used much more easily than you can find all the places where a piece of code is duplicated (or worse yet, duplicated with slight changes).