There is a law of diminishing returns in testing, and there is the danger that by trying to write too many tests you become discouraged and end up not writing any. You should concentrate on where the risk is. Look at the code and see where it becomes complex. Look at a function and consider the likely areas of error. Your tests will not find every bug, but as you refactor, you will understand the program better and thus find more bugs. Although I always start refactoring with a test suite, I invariably add to it as I go along.
