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 $19.40   9.593%   $21.261   $21.26   $21.25   $21.30  
  Purchase #2 $63.71   7.450%   $68.4564   $68.46   $68.45   $68.50  
  Purchase #3 $159.62   7.826%   $172.1119   $172.11   $172.10   $172.10  
  Purchase #4 $65.44   5.705%   $69.1734   $69.17   $69.15   $69.20  
  Purchase #5 $8.47   11.496%   $9.4437   $9.44   $9.45   $9.40  
  Purchase #6 $237.88   3.677%   $246.6268   $246.63   $246.65   $246.60  
  Purchase #7 $57.61   3.102%   $59.3971   $59.40   $59.40   $59.40  
  7 purchase totals = $612.13       $646.4703   $646.47   $646.45   $646.50