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 $128.28   6.730%   $136.9132   $136.91   $136.90   $136.90  
  Purchase #2 $190.92   8.254%   $206.6785   $206.68   $206.70   $206.70  
  Purchase #3 $125.69   6.399%   $133.7329   $133.73   $133.75   $133.70  
  Purchase #4 $54.58   7.843%   $58.8607   $58.86   $58.85   $58.90  
  Purchase #5 $62.74   10.819%   $69.5278   $69.53   $69.55   $69.50  
  Purchase #6 $127.44   5.451%   $134.3868   $134.39   $134.40   $134.40  
  Purchase #7 $245.50   6.313%   $260.9984   $261.00   $261.00   $261.00  
  7 purchase totals = $935.15       $1,001.0983   $1,001.10   $1,001.15   $1,001.10