I have been pondering for a while a means of stabilizing bias cut edges when it is imperative that they been sewn without any stretch. I thought of sewing beside the bias cut line just before cutting, but that might introduce puckering.
But a reason to make a farthingale just recently gave me a chance to try an idea I had. A farthingale is pretty much four rectangles, two of which are cut into triangles (using a bias cut). I ripped the rectangles (to be sure I had the grain true) and then laid the rectangles-to-be-cut-into-triangles out on a grid, to make sure I was keeping the grain as true as I could. I then ironed on a 1 inch wide strip of light-weight iron-on interfacing right over the bias cut line. THEN, I cut the bias line.
It worked! I had no dificulty sewing those bias edges to the on-grain edges without puckering or streching. The seams IS a little stiffer than it would be, but since I flat-felled it, it is just bulkier to begin with. And the interfacing seemed to make the flat-felling easier, as the seam allowance didn't try to deform away from me.
(no point in pictures as it was white on white.)