Valve Spring Installed Height Variation

I'll install them however it is suggested. I don't know enough to debate it, I just thought the spring cups with their rounded edges may have been a better material to rest on the aluminum. I first thought that you didn't want to put the shims between the spring and cups because you then move the spring up and out of the cup by the thickness of the shims.....Then I figured that if I had to shim these .030, that isn't that much and should not be an issue.
I'm open to whatever is suggested. I want to learn and to do it right.