For most of my projects I don't. Some of them have some rules listing test x depends upon y, but for the vast majority of my projects modify file X and only test X is run. This catches a huge non-insignificant amount of regressions. Really it comes down to effort. I can either A) try to remember to always run tests before committing or B) Have a basic hook that takes minutes to write/install that will always run at least one test and catch near all regressions.

I tried to do A for years. Most of the time you run the tests, but not always. And heaven help you if you are on a team. There will be someone who never runs tests. And you will end up having to schedule a chunk of time for regression fixing before every release. So to answer you question who cares about when file X uses file Y.