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As far as I know, shimming the overhead cam valve buckets is to make sure the cam lobe is touching the "top" of the valve. This means that the cam lobe is within so many thousandths of an inch away from the valve. The reason that you should do this is because if they are out of spec, you will hear a lot of head chatter (cams slapping valve shim) as well as you won't have the same EXACT cam spec. This is because the cam will move into the empty space, not moving the valve, so some lift will be lost. A by-product of head chatter is a possible pre-mature wear on the cam, also killing lift and duration.
As for the art and science, what you need to do is bolt down the cam with the lash caps, or cam mains or whatever you like to call the hold-downs, and then measure the cam-valve clearance with a feeler gauge. If it's out of spec (manufacturer spec) than add a shim. Also be sure the is SOME clearance or the valves might not be completely closed.
Is that what you wanted??
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DS
1992 V6 3VZ-FE Camry, 5 speed!
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