On the Codeception blog there's a new post that advocates getting on the testing ship even if the project you're currently on isn't using tests (or TDD).

In this blogpost we will try to figure out how to get faster into the testing. What tests to write at first? Let's say we already have a project and we didn't practice TDD/BDD developing it. Should we ignore testing at all? Definitely no. So where should we start then?

They suggest a three-tiered pyramid approach - UI at the top, Integration testing in the middle and Unit testing as the foundation. They talk about the times when testing doesn't make sense, like when your application is based on a third-party tool (like WordPress or Drupal). They recommend starting with functional testing and working your way back down, especially if your framework supports it. Obviously they encourage the use of Codeception for it, but also recommend even something like Selenium tests if nothing else.