This is a question I sent to Connecting Threads, and below is their response. I just made my first quilt without prewashing fabric and it turned out the same as if I prewashed. So I guess it is your choice.
My question - "I have ordered several kits and charm packs and I have a question about washing the fabric prior to using it in my quilts. I usuall wash fabric and dry and press fabric I am going to use in a quilt projects. My question is on the charms....if I wash them prior to using them they would shred in the wash, so I would think that I have to use it not washed. HOw does that work? Will it work? Thanks for your help. Jo"
Connecting Threads response - "You are correct, you do not want to wash Charms or strips, they will fray to nothing. The new rule of thumb now with these newer cuts is: if you can not wash part of the fabrics (charms) then do not wash any of the fabrics in that quilt. You can then wash everything together after it is done if you want.
Connecting Threads Customer Service
Phone: (800) 574-6454