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 $21.52   13.874%   $24.5057   $24.51   $24.50   $24.50  
  Purchase #2 $81.57   13.395%   $92.4963   $92.50   $92.50   $92.50  
  Purchase #3 $127.34   5.725%   $134.6302   $134.63   $134.65   $134.60  
  Purchase #4 $115.70   3.243%   $119.4522   $119.45   $119.45   $119.50  
  Purchase #5 $46.98   7.483%   $50.4955   $50.50   $50.50   $50.50  
  Purchase #6 $53.25   6.501%   $56.7118   $56.71   $56.70   $56.70  
  Purchase #7 $29.71   10.158%   $32.7279   $32.73   $32.75   $32.70  
  7 purchase totals = $476.07       $511.0196   $511.03   $511.05   $511.00