OK, showing my age. In HS Home Ec we learned to straighten fabric by tearing. It will only tear on the cross grain, not on the bias. Then to make sure the grain line was straight, with 2 people, we stood with the fabric folded in half, selvedges at the top holding the 2 corners of our end of the length of fabric. Keeping the selvedge edges even, we shifted the fabric until it hung straight with no wrinkles. Then, if one side was longer than the other, we dropped that corner and pulled the short side away from one another (like a tug of war) . Rechecked it and worked until the fabric was on grain. This probably will not be too successful on poly blends or any man made fibers that may be heat set in the mfg. process. Not sure why this still isn't done. But no one teaches serious sewing any more. (Do I sound like a grumpy OLD person. tee hee).