Since the fabrics are mostly cut into rectangles that are 6″ by 10″, I don’t pre-wash. I pre-rinse in the sink, roll in a towel, dry and then use magic sizing when I press. The debate about pre-washing continues and I will put in my opinion —
This project is a perfect example of why I pre-treat my quilt fabric. There are at least 12 fabrics that have been stacked and cut the same size for each block. After a nice rinse, they are each a different size and / or shape. Some shrink in wierd ways, some turn into parallelograms. I prefer to have this happen before the quilt is constructed. If you like to steam while piecing that can also cause a shift in the fabric if it has not been pre-treated.
I was once told by someone who had worked in the garment district in NYC that when you see how the fabrics are wound on the bolts it is no wonder that they become stretched and misshaped. A prewash — or even a quick spray with water — helps them bounce back to their original shape.