You should test the thing that makes you money first, and should delay testing supporting functionality. For instance, if I am writing a document converter, the thing that makes me money is the AST -> AST conversion. Testing that should come before testing parsing (bytes -> AST) and rendering (AST -> bytes).
The place where you make money is the place that will have the largest demand for new and changing functionality. And where things change the most is where you need tests to protect against regressions.
The place where you make money is the place that will have the largest demand for new and changing functionality. And where things change the most is where you need tests to protect against regressions.