I don't know about the new stuff, but the old embroidery floss shrinks in the dryer, and puckers up all that beautiful handwork - ask me how I know! Seriously, I would at least swish all the charms around in warm water, maybe twice to get any sizing out, then dry them damp and iron them on hot to get all the shrinking done ahead of time. I wonder if you can pre-wash the floss? Or is the new floss pre-shrunk?
This is an interesting post. I haven't done any hand embroidery for years as I have arthritis in my hands, but I'm finding that a fish oil supplement (about 3-4/day) seems to help. I'd like to try it again, but I don't want to make something and have it ruined by washing. Maybe pre-make a couple of sample blocks, trying different ways of prep to see how they do.