I generally take a middle road - I write code that is unit-testable, but I rarely take the time to write exhaustive tests. When bugs arise, I start writing test cases in various components until I find them. Thus, the debugging effort is what grows the test coverage. Of course in a vacuum it's better to have the cases earlier rather than later, but I like this approach as a speed/quality compromise. The key to making this work is designing in a test friendly way - that is the true art.