I have never heard that. I show slides of Foster when I am teaching figure drawing. I emphasize that Foster drew Prince Valiant for forty years and if he did draw verbatim from models, he had to have run into problems for the model would age. I do wonder though if he ever utilized models. Raymond made no bones about it. There are several published photos of his models besides the drawing he did from them. Frazetta says he never used them and I imagine he didn't in his later years, but I have seen drawings he did for skin magazines in the 1950's where he worked quite closely from models. Jeff Jones took photos of people and did entire strips from the photos, though I have also seen drawings he did utilizing no models at all. Ditto for Williamson. I have seen him draw with nothing in front of him, where it looks like he had a model.
In spite of what Foster has said, I wouldn't be surprised if he utilized models now and then, though I would be shocked if he dressed a paid model as Prince Valiant, took photos then got down to work.
Jeez, granfalloon, that longer post above might be one of the most thoughtful, best written things I've ever read on Comicon.