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 $100.27   4.545%   $104.8273   $104.83   $104.85   $104.80  
  Purchase #2 $19.62   7.480%   $21.0876   $21.09   $21.10   $21.10  
  Purchase #3 $139.28   10.714%   $154.2025   $154.20   $154.20   $154.20  
  Purchase #4 $124.91   5.231%   $131.444   $131.44   $131.45   $131.40  
  Purchase #5 $15.76   8.753%   $17.1395   $17.14   $17.15   $17.10  
  Purchase #6 $217.72   4.265%   $227.0058   $227.01   $227.00   $227.00  
  Purchase #7 $25.56   11.136%   $28.4064   $28.41   $28.40   $28.40  
  7 purchase totals = $643.12       $684.1131   $684.12   $684.15   $684.00