Here′s a test to find out if you′re really achieving that perfect quarter inch seam allowance:
The perfect seam allowance is actually
a SCANT quarter inch. This is due mostly to the fold of the
cloth over the seam allowances. Thicker piecing thread can give you a
thicker "fold over" allowance when pressing. To get the "scant" part,
actually sew a couple threads narrower than a true quarter inch if
The general rule is to never sew over an unpressed seam. However, during block construction there is a danger of distorting the bias if seams aren′t carefully pressed. You may find finger-pressing faster and safer than conventional pressing. Just turn the seam in the proper direction, and use the heat of your fingers to press the seam. Just the part of the seam you are about to sew over needs to be pressed, not the entire seam. Be gentle and do not pull or distort the fabric. Pin and sew the next section of the block. After the block is completed, or at least after a section has been completed with no bias sections on the outside, press with an iron in the conventional manner.