Pressing makes a difference in your finished block. If you routinely press flat first from the wrong side and then turn over and press very carefully from the right side you should be able to improve your accuracy. The pressing technique is just as important as using a scant 1/4" seam. Also check the angle from which you view the fabric under the foot. Sometimes just raising or lowering your chair height results in a view that improves your seam allowance accuracy.