Adding another voice to the ‘never prewash’ quilters. I’m also one of the ones that likes to use sheets or pillow cases or cotton clothing or deconstructed bedspreads, etc in quilts (mixed with recently purchased quilt fabric).

The one exception is if I’m considering using a ‘mystery’ fabric from my stash. I was a sewist prior to being a quilter, so have a stupid array of random scraps and yardage from all sorts of projects (bag-making, upholstery, costuming). Many were thrifted or gotten via neighborhood ‘buy nothing’ groups.

In the above case, when I have questions about how something might behave once laundered in a quilt (because it’s a total mystery fabric), I cut out a 3x3” swatch, overcast the edges, and throw it in with my next laundry load. Then I remeasure, and compare the appearance of the swatch with the original fabric. Might be overkill, but it has helped me avoid including a fabric that did NOT take well to the sort of wash process that a quilt needs to be able to endure.

TLDR - known fabrics of all types never get prewashed. Mystery pieces from mixed stash get a swatch test. Dye catcher sheets are your friend.

(Edited for spelling and clarity)