I think scale models can be quite helpful in designing and building rods from scratch and I highly recommend them. With the one caveat being you are not going to get an exact replica of the original body. But most scratch builders are not after an exact copy of a deuce or a T or whatever rod they are building. What they are after is the basic dimensions and proportions. Scale models can also be used to determine if certain frames will be usable under certain bodies.
So 302 is correct about the limitations of using scale models for precise detail, but for scratch building a basic body - I think they are the next best thing to having an actual body sitting in your garage.