Unless I'm not understanding, no matter how you cut them out, any triangle is going to have a bias edge. If I were going to immediately sew them together to make a square then true, there would be little chance to stretch them. Unfortunately, my triangles are being used to fill in around the four edges of a four-patch that are being placed on point (like a diamond).
Would starching the fabric before cutting them into triangles limit the amount they might stretch?