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 $138.48   13.695%   $157.4448   $157.44   $157.45   $157.40  
  Purchase #2 $57.99   3.165%   $59.8254   $59.83   $59.85   $59.80  
  Purchase #3 $64.86   3.382%   $67.0536   $67.05   $67.05   $67.10  
  Purchase #4 $11.65   6.786%   $12.4406   $12.44   $12.45   $12.40  
  Purchase #5 $215.60   13.791%   $245.3334   $245.33   $245.35   $245.30  
  Purchase #6 $241.59   7.797%   $260.4268   $260.43   $260.45   $260.40  
  Purchase #7 $27.66   3.422%   $28.6065   $28.61   $28.60   $28.60  
  7 purchase totals = $757.83       $831.1311   $831.13   $831.20   $831.00