Great question. Does anyone have an answer?
Im thinking Its always better when it's a stripped receiver. The machine that does the engraving isn't very large. All three of my receivers were stripped when I had them done. It's been a good while since I last visited and this thread reminded me that I have two newer approvals that need engraved.
Just to be 100% sure, call the man and ask him personally. Paul is a super personable but professional guy. He even went as far as machining a custom suppressor mounting AK adapter from scratch that no one else would even consider.