Why skimp? Metal prep and wood prep are the most important steps in finishing any gun. You would never think about finishing a stock that had not been sanded and filled so why would you engrave a gun without first getting the metal in first class shape?
You can have ten grand of engraving look like crap if the metal is not well and properly prepped before the job and then finished afterwards. Metal prep is time consuming but not that high tech hard to do. It does take time and effort to do well. Might be another matter if you were just trying to pick up the old engraving or were on a tight budget on a gun that you were fixing up to sell.
