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   5.186%   $18.923   $18.92   $18.90   $18.90  
  Purchase #2 $168.71   12.807%   $190.3167   $190.32   $190.30   $190.30  
  Purchase #3 $177.33   7.667%   $190.9259   $190.93   $190.95   $190.90  
  Purchase #4 $115.30   3.560%   $119.4047   $119.40   $119.40   $119.40  
  Purchase #5 $14.49   11.180%   $16.11   $16.11   $16.10   $16.10  
  Purchase #6 $135.40   3.859%   $140.6251   $140.63   $140.65   $140.60  
  Purchase #7 $146.52   11.647%   $163.5852   $163.59   $163.60   $163.60  
  7 purchase totals = $775.74       $839.8906   $839.90   $839.90   $839.80