There was a long contentious discussion about the Blu Ray of The Wizard of Oz at the Home Theater Forum sometime back concerning this. Some people were dismayed that the string making the Cowardly Lions tail has been digitally removed. It was argued that on a 1939 release print, audiences would never have been able to see the wire, because the filmmakers knew the film grain would conceal it.

I'd wager you'd probably not see the wires on the Learjet model on an old print of Goldfinger, either. Lowry painted them out for the UE master.

One article I read about matte paintings as a kid specifically mentioned film grain helps them blend into the live action.