Do you have data supporting the 99.9%? I suspect that's high; there are buildings within five blocks of where I'm sitting that have a zipcode that crosses a state line, and I don't think it's the only state-crossing zip in my area (Washington, DC, for what it's worth). In any event, even if it's accurate, it doesn't take that much traffic for a .1% error rate to start to add up to non-trivial number of users, especially because that .1% is going to have a noticeably-jarring experience, as opposed to the alternative (collecting all fields) which everyone is already completely accustomed to.