Depending on the current finish it might not be necessary, but would probably be a good idea anyway. Sandpaper is cheap. I would also mask the faces where the stem meets the bar as well as the area that the fork steerer goes in. Those are made to a specific diameter and you want to keep them that way. The masking on the bar area should also cover the bolt holes there, but I would also cover or fill with wax/grease the threaded bolt holes on the fork area. Might make getting the bolts back in there.