I pulled shims out of 4-5 B230's at a yard about 8 yrs ago. Haven't needed to buy any shims except for when I had to buy new shims because I put an aftermarket manufactured cam in, but have been using the used shims ever since, still a few left of various sizes.
Most of the shims I pulled were very close to each other in size, both within an engine and between the different engines I got them from. I would say go and do it.
But when I get to within 2-3 thou of the size I need, I hand lap them to the exact size needed anyway.
Also note that you should use calipers with a broad flat face to measure them. A single point at one location will usually give you an erroneous reading. You need to find the highpoints on the shim across where the lobe contacts. I found this to be true even with supposedly new shims where the machining causes a dish in the center.