If you have concerns about the actual rings themselves possibly being out of alignment in a matched set, considering how small such an amount would likely be if it existed, then considering shimming a scope or ring would seem foolish. Shimming (either scope or ring) would physically create more misalignment on the scope tube in relation to ring saddle than would ever likely occur from mis-machining of the rings themselves. That is why I asked the question. Orangeit noted concerns over 2 piece ring alignment above. I don't shim anything, ever. Many do, and are happy with the result. I use Sportsmatch adjustable rings when correction is needed to avoid such possible loading of the scope tube and 2 piece rings have always worked fine for me. If one piece mounts give a shooter more confidence then by all means use them.
As to reasons to shim (or use adjustable mounts), it could be as you note to correct a severe misalignment of barrel and rail where you have neared the limit of scope adjustment, or to correct excessive droop, or just to keep the scope near "optical center" when mounted.
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