Generally speaking, yes, but it's nice to have the option. I've often encountered situations where writing tests is a good part of either developing a complex s

Generally speaking, yes, but it's nice to have the option. I've often encountered situations where writing tests is a good part of either developing a complex system, or debugging it when it goes wrong. As a random example, say you're making a data-structure of some sort, and you need to write some internal mechanism to mutate it (balancing a binary tree, resizing a hash table, whatever). If that internal mechanism is somewhat complicated, or gets called many times in public call (or even recusively), it's nice to be able to write tests for it specifically, to make sure you got this critical piece of the puzzle right. Writing a test for just the public interface would probably catch that SOMETHING is wrong, but not what part of the code. It's basically a version of sprinkling your code with assertions to make sure you've got your invariants covered.