Hi Bill,
No you are wrong. It all depends. Small inlays like barrel key escutcheons that are pinned in place need to have the pin head filed down and finished. That could damage engraving so I usually (not always) engrave those after they are installed. However, patchboxes, sideplates, thumb pieces, trigger guards, etc are engraved before installation. If I can engrave an inlay without risking damaging it during installation, I will always engrave it before putting it in place. That said, I have no fear of engraving an inlay in place if I have to. It just is more awkward in the gun because I cannot put it in my engraving block.
dave