If cotton threads are going to shrink, they will do it the first time water or steam are applied. So, the question is when do you want this shrinkage to happen, before you cut, when you press the various sections, or, when you wash the quilt. Use care when you press to minimize distortion cause by pulling fabric out of grain. Fabric is treated with various finishing products to minimize hard set wrinkles like we used to have in our clothing. That was the reason we sprinkled items to iron and let them set for a while before ironing. I don't pre-wash unless I have concern about color retention. I do not like to use starch (or other conditioners) because it does not allow the fabric to behave as it naturally will. I use care in cutting and handling the fabric throughout the process and always achieve good results. You cannot make fabric permanently behave in an artificially induced process. It will always revert back to its natural state or it will cause stress related deterioration.