valve
There are at least 2 different styles of hardened valve springs cups as far as location centering. If the cup has a wall/ring around the outside of the spring, it is meant to be located by the head materal outside of the cup. If this material is not tall enough to hold the cup, then look at the type that has the ring/wall to the inside of the spring. it is meant to be centered by the guide.
In either case, the cup is hardened for the purpose of protecting the head from wear of the spring. Spring shims are too thin to withstand the spring even if they are premium materal and heat treated. That is why they should be installed under the cups only. Why take a chance they may or may not be eaten up and pass thru the engine oiling system?