Width shrinks more than length I believe. So, when you have unwashed charm squares, the shrinkage could be very odd in the finished project I would imagine. I pretty much wash everything first for this reason. I've avoided the issue with the packets as I don't use them or kits much for this reason and for the extra $s they cost. I've had real problems using some precut fabrics in sampler classes too.
Think I would really wet them good with water or starch and steam press them. Then when I'd sew, I'd then treat every 'shorted side' of the square as being a scant cut I guess, if that makes sense. That's not very practical but I've been forced into worse I guess. BTW, one thing I've always hated with kits: Borders cut from the width of the original fabric that results in sometimes wavy borders for my project. I just don't understand why they can't be cut from the length of their yardage for these kits! After all, they are preparing numerous kits at a time and aren't like the individual quilter who has to think more than twice about buyin the extra fabric!
OK, here's Mary Ann getting off her soap box now...