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 $11.41   11.873%   $12.7647   $12.76   $12.75   $12.80  
  Purchase #2 $33.50   8.417%   $36.3197   $36.32   $36.30   $36.30  
  Purchase #3 $167.72   9.389%   $183.4672   $183.47   $183.45   $183.50  
  Purchase #4 $37.46   8.309%   $40.5726   $40.57   $40.55   $40.60  
  Purchase #5 $250.36   11.443%   $279.0087   $279.01   $279.00   $279.00  
  Purchase #6 $143.94   9.496%   $157.6085   $157.61   $157.60   $157.60  
  Purchase #7 $115.68   5.290%   $121.7995   $121.80   $121.80   $121.80  
  7 purchase totals = $760.07       $831.5409   $831.54   $831.45   $831.60