If I don't prewash my fabrics I get itchy due to the chemicals and dyes used. I use a lot of batiks, they bleed dye and must be washed before using.
I sewed garments before I learned to quilt. I still make shirts for my DH and he asked if his shirt would shrink in the first wash. I said, no because the fabric was washed before it became a shirt. He was relieved that it wouldn't shrink.
Sharon in Texas