As far as I know, you can only shim pedestal rockers. But even then, shimming them is not to correct the geometry. It's to put them at correct height.
The geometry has to be correct and that's done with pushrod length. If the pushrod is too long or too short, the rocker arm tip will ride on the outer edges of the valve stem, which will eventually wear out the valve guides. The pushrod needs to be the correct length so that when the engine's rotating, the rocker arm is centered on the valve stem. You can test this by covering the valve stem tip with a dry erase marker, properly install the rocker arm, and roll the engine over a few times. It'll rub off where the rocker is touching. If the wear mark is on the side of the pushrod, it needs to be longer. If it's on the opposing side, the pushrod needs to be shorter. Centered is just right. A pushrod length checker is a pretty cheap tool for this.
Shimming comes into play on pedestal rockers because they have a set height based on the "pedestal". You just bolt them down into the head and there is zero adjustment on where the rocker sits. If the pedestal is too short, it'll compress the valve spring and/or lifter. Shims will let it sit up a little bit higher and relieve that.
Stud style rockers are simply held in place by sitting on top of the spring and push rod, so they "float". There's nothing to shim on them. Adjustments to the geometry are only
made with pushrod length. The nut on top moves rocker up and down. If it's up too high, there'll be slop, and the parts will clatter and be loose, which will eventually wear them out. If they're too tight then the valves will be held open even when they're supposed to be closed, which will cause the engine to run poorly (AND have low compression since the compression will seep past the valves
). If they're WAY too tight, they'll bottom out the lifter and prevent it from coming up all the way. This will bend pushrods and/or chew the lobe off of the cam. When it's the right height, the valve will be fully closed when the lifter is off of the cam lobe and the slop will be taken up by the hydraulics in the lifter.
The original "torque-and-go" rocker arm studs had a ledge on them that was the correct height for a 100% stock engine. When the nut hit the ledge, they wouldn't tighten anymore, and the rocker was set to the perfect height. Great for an assembly line, and for a completely stock engine that's never been rebuilt. But quickly becomes a pain once you rebuild things, use different rocker arms (like roller rockers), and shave down the heads and block since that set height can't be adjusted, and is most likely wrong.