Yes, that's the bad news about fabric - it stretches. But the good news is... fabric stretches. I make pieces fit at the sewing machine and they always end up flat. I make sure there is enough seam allowance at the top of the geese so the point is not cut off and that the sides are perfect diagonals so there are nice points there as well. I always sew my geese (or other units) to an exactly measured piece and again, work it to make it fit - so no flairs in borders. Being aware of the stretch is the first step in fixing the problem.