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 $209.46   4.179%   $218.2133   $218.21   $218.20   $218.20  
  Purchase #2 $240.85   13.442%   $273.2251   $273.23   $273.25   $273.20  
  Purchase #3 $13.27   3.293%   $13.707   $13.71   $13.70   $13.70  
  Purchase #4 $36.97   12.655%   $41.6486   $41.65   $41.65   $41.60  
  Purchase #5 $127.65   10.637%   $141.2281   $141.23   $141.25   $141.20  
  Purchase #6 $126.53   6.211%   $134.3888   $134.39   $134.40   $134.40  
  Purchase #7 $124.58   9.338%   $136.2133   $136.21   $136.20   $136.20  
  7 purchase totals = $879.31       $958.6242   $958.63   $958.65   $958.50