I use autotest for this - it notices files being altered and runs relevant parts of your suite. If something goes wrong it'll keep trying that test everytime you save a file, until it works. I think it's fantastic.
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.