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.