Valve Spring Installed Height Variation

I'll be checking the installed height on the next 2 engines that I'll be working on so I'm trying to wrap my head around this.
Using nothing but common sense with no experience doing this, I'm guessing the following:
* The top of the valve stem is a fixed measurement since the valves are against their seats.
* The "Installed Height" is the height of the spring as measured from under the retainer at the top to the point where the spring sits on the head or the steel pocket it rests in.
* Any shims that are added are placed under the spring at the bottom. Cast iron heads have no spring pocket seat cups, aluminum ones do. Do the shims go between the spring and the cups or the cups and the head itself?
* As shims are added, this adds tension to the spring and increases it's rate. Correct?