I am in total agreement with Roshyf, in fact I am a bit confused by the subject as I have always thought, and read and been taught and told that you should always use the straight thread of the fabric.
One of my favourite suppliers, and early quilting teacher, used to have a saying, I live to rip and cut. She had an Internet business and she taught me that you rip a piece of fabric, to get the "grain" of the fabric straight, and then cut. I have never had problems with warped blocks or borders, which IMO must be cut from a straight thread fabric.