Not necessary, for engraving. It's actually smarter to engrave it first, before you send in paperwork - Imagine filling out your paperwork, sending it in, getting your stamp. You send off your lower for engraving, and they Eff it up (mis-spell something). NOW, you're screwed.
Alot of guys send it out for engraving, get it back, THEN do the paperwork, for that very reason.
Having a stamp is not a necessity in order to get it engraved.