The accuracy gained from the Perfect Corner Ruler method far outweighs any problems arising from the fill-in triangle’s bias edge. The “fill-in” triangle is already over-sized so getting it to fit does not require stretching it. The only issue we’ve had is that it “wants to move out of place” so one should pin the fabrics, as the video warns. Also, as the triangle is placed below the “base fabric” [block], one does not have the opportunity of stretching it. Although I might be “biased” because my wife created it, we have seen marked improvements in piecing, received many thanks, and not a single complaint from anyone who has tried it.