I never pre-wash pre-cuts. I did it one time a few years ago and it was a disaster. I never have since and have never had an issue. The shrinkage you get when you wash the finished quilt makes the quilt look all the lovelier.

I have also mixed prewashed yardage with non prewashed precuts and not had any issues with the finished quilt. Once the quilt is made and washed you can't tell which fabrics were prewashed and which weren't. It all comes out fine.