An old speed guy once told me to use the same washer plug after plug.  It tends to take a set to the shape of the head.  The other thing he insisted upon was using a die to chase the threads of the plug before using it.  It takes a 1/4-32 die to run the threads of the glow plug.  This cleans up the burrs that can be left on glow plugs, and prevents damage to engine heads.