I try to starch and press each seam in order to "preserve" the shape that I have sewn. If one is pressing to try to "manipulate" the fabric to get a block into being square or to be a bit larger, etc, remember that it will always return to the original shape when washed. It is really good to measure and square up as you go. Waiting until a 12" block is completed is usually too late to guarantee that it is squared.