Very common practice on dragster engines, I once worked at a dealership with the crew chief for a UK team and he had a box of assorted thickness shims in his kit and I asked what they were for. he said its all about flame spread and alignment of the electrode spark in the most efficient segment of the combustion chamber, I started yawning after that.
Apparently also used for those at the extreme end of concourse in some countries who want the names on spark plugs aligned. (and yes the judges also check the plug torque to see if anyone's cheating... I would have them all wrapped in neoprene ;-))
