The manufacturer's "typically" will make their products as true to scale as possible, but even so they may have to take some liberties. For example the best readily-available motor may require them to make a shell a little wider than it should be, so their choice would be to make the shell wider or get a new, unique for their application motor and bump the price to the point where it wouldn't sell. Which would you do?
Most of the magazine reviews check out at least the major dimensions againt the prototypes and comment on them. For example, here's a qote from Model Railroader:
"All of the models' major dimensions match drawings of E6 A and B units published in the March 1983 issue of Mainline Modeler. The rakish nose, which slants at a sharper angle than noses on the E7 and later E units, is well done, but the model has a bit of a sharp angle just above the rear of the anticlimber on each side. This area should be more gently rounded. "
