It's a good idea to record invariants about your code and your problem. Tests are one way, and there are some other ways. It's a good idea to have the checking of invariants be automatic somehow.

Any assumptions about the output of your code that you make but don't record in a checkable form, are something that could potentially be silently broken by a change in the code. And then you're wasting your time debugging rather than solving the problem you're working on.