I'm hung up on this statement, and now you've got me wondering about something. I even went looked at my paperwork because of it (after all, I was fishing for out-the-door prices including tax, then put $9k down).
Tell me if I'm wrong, or if it varies from state to state, but as far as I know, sales tax is figured on the final sales price of the vehicle before trade-in/down payment. At least that's what my paperwork is showing. I can't even remember how it was done on my previous vehicles.