My mom, who taught me *almost* everything I know about sewing, doesn't worry about perfect cuts. When stuff absolutely must match, she makes a cardboard template for finished size of shape, butts it up against the last seam she sewed, draws a pencil line for the new seam, and sews there.
Does it take longer to sew pieces together that way? You betcha. But it takes less time to cut the fabric, and less time ripping out imperfect seam allowances. Works very well for simple shapes (squares, triangles, hexagons).