jump in with your scraps- create a block---wash it & dry it & see if there's a problem---but remember that batting, & quilting will control some of the shrinkage too- i've made lots of string quilts- have never had any shrinkage problems with the finished product- even when some of the fabrics were prewashed & some were not. i mix them up all the time- if mixing flannels/home spuns in that have a looser weave & tend to shrink more i'd be sure to wash them- but any others i wouldn't bother.