I don't know the first thing about "etiquette" in this matter, but from my point of view you are buying the machining services, by the hour with some minimum, probably, of this gunsmith, so there should be nothing wrong with taking in the sights you want to have them installed. He can quote you the price he wants. If he is offended by this, find you a different gunsmith who wants to work. I think the one I use would be happy to do this work. There should be no reason he needs the whole pistol; the slide should be sufficient.
RonF