Michael Hartl on when to write tests based on his experience

• When a test is especially short or simple compared to the application code it tests, lean toward writing the test first.
• When the desired behavior isn’t yet crystal clear, lean toward writing the application code first, then write a test to codify the result.
• Because security is a top priority, err on the side of writing tests of the security model first.
• Whenever a bug is found, write a test to reproduce it and protect against regressions, then write the application code to fix it. Lean against writing tests for code (such as detailed HTML structure) likely to change in the future.
• Write tests before refactoring code, focusing on testing error-prone code that’s especially likely to break.