I'm pretty much the one man code shop for our startup and I still write a lot of tests. The way I think of it is this: if something is tricky enough that I need to verify it in the repl, may as well capture that validation in an automated test. The trickier, more painful tests to setup are integration tests that make sure everything is hooked up correctly, from the datastore layer to the handler to the template arguments etc. I went through the pain to set this up so that we at least have smoke tests, e.g every page is visited with some data populated to make sure nothing blows up.

A good reason to write tests beyond QA is to verify your code is at least somewhat modular - being able to get code under test ensures at least one additional use beyond being hooked into your application. For that reason, I would recommend having at least one test for every module in your code. It also makes it easy to write a test to reproduce a bug without having to refactor your code to be testable after the fact.