Dan,
You can tape it if you like, but most times you have extra wood on each side of the butt piece, so you have to rasp anyway to get it flush.
After you mark the outline of the butt piece on the stock, you can make a series of horizontal cuts from the end grain, towards the outline. Instead of taking off one big hunk, you get a series of little pieces. It's a little easier to follow your butt piece outline that way. Using a thin blade saw also helps. Don't be overly concerned about a smooth cut because you'll be doing lots of rasping to fit the butt piece to the stock. Hope that helps...
Ed