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 $17.99   4.301%   $18.7637   $18.76   $18.75   $18.80  
  Purchase #2 $107.94   4.110%   $112.3763   $112.38   $112.40   $112.40  
  Purchase #3 $141.37   11.298%   $157.342   $157.34   $157.35   $157.30  
  Purchase #4 $156.94   12.280%   $176.2122   $176.21   $176.20   $176.20  
  Purchase #5 $140.35   8.707%   $152.5703   $152.57   $152.55   $152.60  
  Purchase #6 $50.45   10.532%   $55.7634   $55.76   $55.75   $55.80  
  Purchase #7 $45.27   7.692%   $48.7522   $48.75   $48.75   $48.80  
  7 purchase totals = $660.31       $721.7801   $721.77   $721.75   $721.90