I prewash everything. I do this to check for bleeding colors and shrinkage. I didn’t used to wash precuts until I finished one quilt top that was quite complicated. Of course all of the fabrics had been prewashed EXCEPT for the jelly roll I used for the places where I needed sashing. Mind you, it took me 3 months to piece the quilt top alone (it was king sized, 108” X 108”). I did the backing, quilting, and binding. All total, it took me just over 5 months to finish (could have done it faster but was working full time.). Then I washed it in the super large washer at the laundromat. I used tepid water, small amount of Ivory liquid soap and 3 color catchers. Dried it on low until it was still damp, and tossed it over two laundry lines in the back yard. When I went out to inspect it, I was horrified to find that the darker colors from the jelly roll had bled all over the lighter colored blocks. Plus the fabric from the jelly roll shrank badly, warping the entire quilt. I went into meltdown mode. All that time, effort and materials that went into the quilt were wasted. It took me 6 months before I could even go near my sewing machine. Ever since then, I buy the fabric and cut out my own ‘precuts’ from prewashed fabric, or I use Fat Quarters that can be prewashed. But that’s just me, and I’m certain other people have their own way of doing it.
MacThayer