No at the start of a project. Yes for refactoring.

I used to write tests for all pure functions because they were the easiest tests to set up. They are also the easiest to debug so the test didn't really help unless you're checking for type signatures.

I think that implementation tests are important but I found that I suck at figuring our how to set up a test before the actual implementation. So I do them after the fact and judiciously.