I'm amazed at the number of people that don't test at all but also at those that blindly believe that because they're told to test they should always do it. I think it's important to pick and choose when it makes sense for you to test, when you are prototyping then you might not need to, in some cases though it's actually quicker to write something to test an output than it is to keep trying it another way.

The real skill in testing is knowing when it should be done and how. It's good article and ultimately until you have tested you can never know when it's write to make use of testing and TDD.