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 $47.42   6.495%   $50.4999   $50.50   $50.50   $50.50  
  Purchase #2 $56.41   11.621%   $62.9654   $62.97   $62.95   $63.00  
  Purchase #3 $84.49   13.593%   $95.9747   $95.97   $95.95   $96.00  
  Purchase #4 $70.61   11.706%   $78.8756   $78.88   $78.90   $78.90  
  Purchase #5 $156.58   6.364%   $166.5448   $166.54   $166.55   $166.50  
  Purchase #6 $250.77   4.475%   $261.992   $261.99   $262.00   $262.00  
  Purchase #7 $181.95   7.841%   $196.2167   $196.22   $196.20   $196.20  
  7 purchase totals = $848.23       $913.0691   $913.07   $913.05   $913.10