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 $19.95   6.424%   $21.2316   $21.23   $21.25   $21.20  
  Purchase #2 $70.93   11.211%   $78.882   $78.88   $78.90   $78.90  
  Purchase #3 $208.68   13.274%   $236.3802   $236.38   $236.40   $236.40  
  Purchase #4 $16.56   8.239%   $17.9244   $17.92   $17.90   $17.90  
  Purchase #5 $16.46   5.750%   $17.4065   $17.41   $17.40   $17.40  
  Purchase #6 $101.47   10.545%   $112.17   $112.17   $112.15   $112.20  
  Purchase #7 $44.56   6.584%   $47.4938   $47.49   $47.50   $47.50  
  7 purchase totals = $478.61       $531.4885   $531.48   $531.50   $531.50