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 $58.52   12.863%   $66.0474   $66.05   $66.05   $66.00  
  Purchase #2 $122.27   9.196%   $133.5139   $133.51   $133.50   $133.50  
  Purchase #3 $242.79   8.130%   $262.5288   $262.53   $262.55   $262.50  
  Purchase #4 $75.53   3.685%   $78.3133   $78.31   $78.30   $78.30  
  Purchase #5 $44.87   5.263%   $47.2315   $47.23   $47.25   $47.20  
  Purchase #6 $15.97   13.158%   $18.0713   $18.07   $18.05   $18.10  
  Purchase #7 $206.51   5.526%   $217.9217   $217.92   $217.90   $217.90  
  7 purchase totals = $766.46       $823.6279   $823.62   $823.60   $823.50