Only if there's never more than 10,000 addresses in a single zip code, which means that if you enforce that, you can force a zip code to appear by building enough house
I think my point is that if you're going to make people learn a 9-digit identifier for their house you might as well make that identifier unique and then that's the only information they need to fill in. Having non-unique 9-digit identifiers feels wasteful.
