I used to wash everything...but hey! I got piecing to do....now I am selective when I prewash....quilt shop fabric I generally do not wash....unless it is red or black...and then I sometimes do a bit of testing by rubbing a white tissue over it and seeing if there is a color transfer...or sometimes, if it is going to be a lot of red or black, I toss it in the washer with a couple of color catchers...one red batik I have, has deeply colored color catchers during three washes...don't know what I am going to do with this one...I also will prewash flannel used for a backing...unless I want a puckered quilt which sometimes I do for flannel quilts....and IF I am using a cheaper quality backing flannel as it will shrink and I want to minimize that if I have pieced the top with good quality flannel...just did that for some scrap quilts I made. I also will prewash if I am using a lot of fabric from a lot of different lines and manufacturers...yep, some shrink more than others. I used to measure my tops before quilting, after quilting, and then again after washing....yep...I could lose a couple of inches after quilting and a couple more after washing....