I do not prewash, nor do I starch. Purchased fabrics usually have plenty of sizing in them. If you are using fabrics from clothing, just be aware of buttonholes, places where buttons were sewn, lines where a hem was folded, etc. and go for it. When I am ready to make a project I find the fabric I want and head for the cutting table and get on with it. I've been quilting for 60 years and never had a problem.