Mixing washed fabrics with unwashed charm packs--work?
 

Cold water wash--with a color catcher sheet... but does the shrinking issue come into play?

I would still wash with warm or hot then hot dryer. I would put the charm packs into a lingerie bag. If you don't have a lingerie/mesh bag use pillow case.

I have used washed and unwashed fabrics, and have never had a problem with uneven shrinkage. Although I think it is best to use all one or the other, it just doesn't seem to work out that way for me many times, guess I am lazy. But I NEVER prewash precuts like charms or jelly rolls, and if I want to use them with other pre washed yardage from my stash, I do so.

If you wash charm packs, expect the squares to shrink and not be square.
I'm with Jeanne, I don't wash precuts...too much work.

If you are worried about colors running, I'd hand wash them by separating them by color and then check for colors running. Then you could dry them by laying flat on a towel or two.

I mix washed/ unwashed fabrics in the same quilt all the time. I've never had any type of negative issue. I prewash fabrics that might bleed ( deeply saturated hand dyes) or if the fabric smells or feels funky. If none of those circumstances are at play I generally don't prewash. I make a Lot of quilts! I've never had something go wrong due to this practice. And, I Never prewash precuts

Oh trust me--I will never wash a precut. I recently bought several Charms from Green Fairy for bags, but there's one line I'd love to make a quilt out of--so I bought some yardage. I'm not washing it--since the Charms aren't washed, but the solid I will use to accent is washed. And with the bags--I often add a washed piece from stash for the inside or bottom--the rest is Charms. And the Charms I don't use for bags will go in Tummy Time Chunk quilts--all the rest is washed scraps. So---I went ahead and mixed them--didn't think it could be a huge issue. I don't buy/use Charms that are bold colors. Thanks for the good news. :-)

If you wash your charms, they won't be exactly 5" squares.