I'm continually surprised at how many shops don't test their software or don't have test plans. The ROI on a test plan is being able to say: "I did my due diligence." When you have a contract on the table you can point to a record of testing to say that every possibility that could be thought of was, a plan was made and tested. Without a formal testing plan you are just hanging yourself out there hoping to see nobody calls you out on your mistakes.
I understand that if you are working time and materials that all the company that contracted you is paying for is your time an any materials you consume. They technically aren't paying for a working product. That seems far too under-handed to me. How could anyone sleep at night knowing that's how they worked?
I personally strive to make certain that everything that I produce first does no harm and second does what's expected and third does what's required. Sometimes I've found that these goals are in conflict. But first I want to be certain what I write isn't worse than nothing or worse that total failure.
I suspect that I might be scratching at the core of some kind of oath.