0.3mm and thicker shims do keep their shape quite well, the thinner ones can be easilly misformed.
Recently at a Dutch shop I did find 0.2 and 0.25mm shims in bags of 100pcs so now it is much easy to build up a clutch with fresh shims. Basically I use only fresh shims when do a rebuild of the clutch wih a fresh shoe and once in a while a fresh spring. For springs I always use the hard ones that gives the clutch more push. I also use Belleville washers as a clutch spring, I have a few hundred of them of which the most still need to be seized down to fit the clutch..