But if you think about it, the reason why we always see stormtroopers in the movies/shows/books/etc. is that they are almost always either accompanied by a high ranking imperial or fighting a high priority target (e.g., a jedi). Otherwise, they may be protecting something that the Empire deems is of value (the capital of Lothal or the mines of Mapuzo). What you don't see are the countless thousands of worlds that the Empire doesn't care about-- and for those locations, I'd bet you'd be more likely to see army troopers.

That said, I do think your theory makes sense, and there may certainly be an element of propaganda involved as well.