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 $229.70   13.863%   $261.5433   $261.54   $261.55   $261.50  
  Purchase #2 $121.34   10.244%   $133.7701   $133.77   $133.75   $133.80  
  Purchase #3 $138.31   3.803%   $143.5699   $143.57   $143.55   $143.60  
  Purchase #4 $161.38   8.762%   $175.5201   $175.52   $175.50   $175.50  
  Purchase #5 $41.31   7.874%   $44.5627   $44.56   $44.55   $44.60  
  Purchase #6 $197.92   7.793%   $213.3439   $213.34   $213.35   $213.30  
  Purchase #7 $10.98   11.739%   $12.2689   $12.27   $12.25   $12.30  
  7 purchase totals = $900.94       $984.5789   $984.57   $984.50   $984.60