I don’t prewash. I wait til the quilt is done and then wash it. I don’t have any chemical sensitivities or allergies and have never had an adverse reaction to the scent or touch of new fabrics. If I suspect a bleeder I cut a small piece off and test it in very hot water. I also don’t have a stash except for some fat quarters I won and lots of scraps. So nothing lying around outgassing. So far after 16 years of quilting I haven’t had a problem.
this question comes up now and then. Like starch. I don’t think there is one answer or a right answer. Just what works for each of us.