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 $104.43   13.189%   $118.2033   $118.20   $118.20   $118.20  
  Purchase #2 $46.55   13.855%   $52.9995   $53.00   $53.00   $53.00  
  Purchase #3 $141.50   4.212%   $147.46   $147.46   $147.45   $147.50  
  Purchase #4 $55.69   5.154%   $58.5603   $58.56   $58.55   $58.60  
  Purchase #5 $6.60   6.379%   $7.021   $7.02   $7.00   $7.00  
  Purchase #6 $249.37   4.690%   $261.0655   $261.07   $261.05   $261.10  
  Purchase #7 $123.73   8.411%   $134.1369   $134.14   $134.15   $134.10  
  7 purchase totals = $727.87       $779.4465   $779.45   $779.40   $779.50