I have only used charm packs one time... It was for a crazy 9 patch class (and the only quilting class I ever had also). It was great to have a nice color combination and not have to cut. But I found I had to do a LOT of squaring of the blocks when I put it together and I was not real happy with the finished quilt after I washed it. Then this week, I needed a few coordinated pieces to try a new pattern so I pulled out 4 of these unwashed charm squares left from that project. I noticed right away that some of them were not square. I had to trim a full 1/8" from one corner on one of them.
So not only possible shrinkage, but possibly the charm squares aren't square. I am not sure I'd buy them again though that was not the context of this thread.