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.
And even beyond that, some governments levy different taxes on different items (e.g., lower tax on groceries), while other nearby governments may levy the same tax on everything.
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.
A tangential, but this complexity is one reason I think that sales tax should largely be abolished in favor of rolling it into income taxes. The only context it makes much sense in is taxing various luxuries. Beyond that, it's confusing, adds administrative overhead, and discriminates towards those with lower incomes.
