Whether you pre wash or not do these etching to all fabrics being used in the project.
Using water in the iron I was told stretched the fabric. With ironing make sure you press and not iron. Ironing again stretches the fabric.
With quilting and the more you do the more you will find there are at least 2 ways to do everything. My friend does not agree with MSQC and Jenny's way of making 4 half square triangles . I like this way but I do a light spray of startch before sewing and cutting to hold the fabric still and stop stretching.
Any small increase in size on every piece alters the size considerable at the end.
One rule to keep......always always put the blade back into its holder immediately after cutting.