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 $217.49   13.321%   $246.4618   $246.46   $246.45   $246.50  
  Purchase #2 $106.44   10.282%   $117.3842   $117.38   $117.40   $117.40  
  Purchase #3 $65.56   11.814%   $73.3053   $73.31   $73.30   $73.30  
  Purchase #4 $232.87   5.863%   $246.5232   $246.52   $246.50   $246.50  
  Purchase #5 $68.72   9.119%   $74.9866   $74.99   $75.00   $75.00  
  Purchase #6 $56.69   4.302%   $59.1288   $59.13   $59.15   $59.10  
  Purchase #7 $86.57   9.643%   $94.9179   $94.92   $94.90   $94.90  
  7 purchase totals = $834.34       $912.7078   $912.71   $912.70   $912.70