Somewhere on one of the sites there is a thread concerning the restoration of a 68-70 Nova. The underbody was done in satin black. Since it is not possible to duplicate under body overspray with bc/cc, after the car was painted they completely taped off the body. The shop then positioned the body as it would have been on the line and sprayed gloss enamel [car is white] along the rockers and rear, perfectly duplicating the factory overspray pattern on the underbody.
That's what I mean. I believe Caudells was the first to do this over 10 years ago on an Orange COPO Camaro. People were shocked.
There are several current threads discussing how to best duplicate factory overspray so I'm thinking it is a trend. You're certainly free to do yours with no overspray at all, nice polished stainless brake & fuel lines, phony looking firewall "graffitti" nice glossy radiator, show chrome bumpers, etc. But it didn't look like that new.
I have to pick on someone a bit. Some years back I was asked to critique a fresh resto on a COPO Camaro. About all I could find was no undercoating in the wheel wells-the factory did all 4 BTW; nice & sloppy. He admitted he could not bring himself to do it. I'll bet to this day it still doesn't have any!