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 $17.40   3.836%   $18.0675   $18.07   $18.05   $18.10  
  Purchase #2 $33.83   5.166%   $35.5777   $35.58   $35.60   $35.60  
  Purchase #3 $132.63   7.788%   $142.9592   $142.96   $142.95   $143.00  
  Purchase #4 $234.54   6.268%   $249.241   $249.24   $249.25   $249.20  
  Purchase #5 $228.92   13.741%   $260.3759   $260.38   $260.40   $260.40  
  Purchase #6 $138.93   10.249%   $153.1689   $153.17   $153.15   $153.20  
  Purchase #7 $221.46   8.684%   $240.6916   $240.69   $240.70   $240.70  
  7 purchase totals = $1,007.71       $1,100.0818   $1,100.09   $1,100.10   $1,100.20