Another current thread post said to put the shim under the seat. This is on aluminum heads.
The shim is hardened steel.
If the I.D. of the shim is a snug fit on the seat rather than a loose fit on the head wouldn't it be better placed on the seat as long as it is not so thick as to raise the valve spring up out of a cup type seat? This way the shim will not dance around between the head and the seat. It also seems to be a more uniform area of contact. Here is a picture I found showing the shim in the seat cup.

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If the shim is hardened and intended to to contact the spring directly that is fine. Some shims are and some are not.

When an engine is running the valve springs move around in circles and dance all kinds of crazy as harmonic frequencies increase thru out the rev range. The shims must be hardened for this.

However, in the other thread, he has an aftermarket VSI spring shim. It is not hardened or intended to contact the spring. VSI makes a .015 thick hardened shim that you have to use on top of them if you do not have a spring cup, it is easily identifiable by it's black color. If you look at that shim he has, you can see places where the anodizing is wearing off it from contact with the spring. From what I understand he hasn't put very many miles on the engine. It won't take long for the spring and spring damper to chew a groove in that shim and put a bunch of metal in the oil.
Will

Thanks, I'm using 1 Comp. .030" hardened shim inside the spring cups with their 26981 beehives to obtain the 1.70" installed height. I examined them after about 3000 mi. and all is well.
A set of the proper hardened shims might be a good idea for Dale1990. They're cheap.