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 $244.52   12.641%   $275.4298   $275.43   $275.45   $275.40  
  Purchase #2 $77.40   7.187%   $82.9627   $82.96   $82.95   $83.00  
  Purchase #3 $90.44   7.776%   $97.4726   $97.47   $97.45   $97.50  
  Purchase #4 $126.28   4.545%   $132.0194   $132.02   $132.00   $132.00  
  Purchase #5 $59.99   5.161%   $63.0861   $63.09   $63.10   $63.10  
  Purchase #6 $127.97   4.339%   $133.5226   $133.52   $133.50   $133.50  
  Purchase #7 $15.67   11.734%   $17.5087   $17.51   $17.50   $17.50  
  7 purchase totals = $742.27       $802.0019   $802.00   $801.95   $802.00