I was also under impression that to get best fit you need to mill shoulders and sides. All four sides make a shoulder or step whatever you call it so bolster or guard lies flat against it.
You probably can't get away with just grinding shoulders instead of four sides especially if longer sides of tang are not parallel since slot sides made on mill are parallel.
You probably can't get away with just grinding shoulders instead of four sides especially if longer sides of tang are not parallel since slot sides made on mill are parallel.