Not doing it when you can is the bigger waste.
TDD helps cover not just making sure your code works, but when done well can also cover other needs:
- if you write your tests first, gives you a better sense of your progress and how much is remaining (e.g. you have 20/40 tests written and passing)
- it gives future developers an example of how your code actually works with an actual example, not just documentation on "how it should be used"
- this one's obvious, but it also gives future developers some assurances they didn't just break your code with a new feature or bugfix (this assumes you wrote your tests well, of course)
- it can help increase your confidence in your code, and reduce stress in your mind about things you might have missed.. if you think you missed something, write a test, prove your code works -- even if it's just to prove to yourself, it's worth it