It goes further than that; a county or even a town can have a separate sales tax. You can have 9% in one county, 10% in a city inside the county, or 8% the next county over.
Further, some governments charge tax on the pre-coupon/pre-discounted price, while others tax only the actual final price.
Further still, some entities like public schools are exempt from paying sales taxes in many states/municipalities. In this case, the price sign would have to show both a pre-tax and tax-included price on it, which may be the best solution anyway.
Regardless, these differences are probably easiest to account for at the point of sale.