This is a very interesting point which I hadn't considered. That plus the fact that I never buy AppleCare+ makes the full-price option a better deal for me than the upgrade program.
Can anyone confirm if you pay sales tax up front, or if it is rolled in to your monthly payments?
For the carrier plans, It actually varies by state as some states allow you to roll taxes into monthly payments however with this program because of possibility that you upgrade before full payoff, it may have to be calculated differently.
With that's said, it shouldn't impact your calculation. Just assume you pay full taxes in either scenario (upfront purchase vs the upgrade program) and they cancel each other out. You don't pay any more taxes under the upgrade program. If you upgrade one time and then hold the second phone for two years you will have paid sales tax twice. Same if you bought a phone, sold it after one year, and bought another and held for 24 months.