Kenneth Fisher thinks about when to test:
To test or not to test. That is the question. Or is it? Shouldn’t we always test? That was the question posted here. It’s a big question so let’s break it down a bit.
My short answer is: test as much as necessary to keep two conditions from occurring: first, that you get fired (or worse); second, that the inevitable bug somebody else discovers in your code is too embarrassing. Kenneth’s answer is better.