I know it's shocking, but I've mixed unwashed charm packs with pre-washed fabrics from my stash, and nothing bad happened!
However, all fabrics involved were quilt shop quality, major manufacturers.
I have had the experience of some other 5 inch cuts from a certain mail order source where the fabrics shrank from steam pressing -- that made me very reluctant to mix them in with other precuts. However, they were inexpensive, so I easily used them for a tote or wallhanging or something.