Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $239.78   8.438%   $260.0126   $260.01   $260.00   $260.00  
  Purchase #2 $229.71   13.765%   $261.3296   $261.33   $261.35   $261.30  
  Purchase #3 $123.62   5.268%   $130.1323   $130.13   $130.15   $130.10  
  Purchase #4 $49.68   12.834%   $56.0559   $56.06   $56.05   $56.10  
  Purchase #5 $241.39   8.418%   $261.7102   $261.71   $261.70   $261.70  
  Purchase #6 $179.62   11.622%   $200.4954   $200.50   $200.50   $200.50  
  Purchase #7 $158.68   4.231%   $165.3938   $165.39   $165.40   $165.40  
  7 purchase totals = $1,222.48       $1,335.1298   $1,335.13   $1,335.15   $1,335.10