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Sales Tax: Rounded Then Totaled or Totaled Then Rounded?
First of all to answer the basic question "Is one method correct? Might it depend on local laws?" Yes it does depend on local laws. Because ultimately the business will have to file forms with the sate/county/city.  These forms are going to ask for the total sales based on the tax category (tax free, x%, y%). Each transaction could have parts that fall into each category. The local taxing authority decides what goes into each category. The local taxing authority also determines how often the business needs to submit the taxes. They can even decide to base the rates used by where the customer lives.  A business is not required to charge directly for sales tax. That is why frequently at sporting events, the price on the menu notes that all sales taxes are included. I suppose not directly charging a sales tax makes the monthly calculation harder, but the state will still get their money. Rounding up at the end of the entire transaction is enough to make sure they collect enough taxes, so they don't have to dip into their profits.