Do you pre-wash your fabrics before making a quilt? I always do. When I first began quilting in 1986, that was what I was instructed to do and I’ve done it ever since.
Cotton fabrics shrink a little and they don’t necessarily all shrink by the same amount since the quality and the thread count varies. Some people don’t pre-wash fabrics because they like the crinkled antique look their quilt gets when it is washed and the fabrics shrink. Sometimes people buy fabric at a store or a conference where they are taking a class, They want to use the fabric immediately and don’t have access to a washing machine. My advice would be to not to mix the washed and non-washed fabrics. Apart from the shrinkage factor, the sizing/chemicals in unwashed fabrics makes them stiffer and less pliable which may cause minor problems when piecing them with washed fabrics.
Another issue is whether or not the fabric will bleed. All fabrics should be color-fast, but some shed excess dye the first time they are washed. If unwashed fabrics are stitched into a quilt and they lose some dye, the dye will be picked up by adjacent pieces in the quilt and damage will be done, e.g. red bleeding onto a white background. Putting a color-catcher sheet into the washing machine with the quilt to pick up any excess dye will help to overcome this problem.
These days, the general quality of quilting cottons sold at quilting shops is high, and shrinkage and dye loss are not the problems that they once were. However, it’s still advisable to be cautious and aware of these issues.