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 $172.40   9.871%   $189.4176   $189.42   $189.40   $189.40  
  Purchase #2 $18.89   6.465%   $20.1112   $20.11   $20.10   $20.10  
  Purchase #3 $71.83   5.273%   $75.6176   $75.62   $75.60   $75.60  
  Purchase #4 $8.53   8.656%   $9.2684   $9.27   $9.25   $9.30  
  Purchase #5 $139.44   13.571%   $158.3634   $158.36   $158.35   $158.40  
  Purchase #6 $118.84   5.665%   $125.5723   $125.57   $125.55   $125.60  
  Purchase #7 $14.46   4.680%   $15.1367   $15.14   $15.15   $15.10  
  7 purchase totals = $544.39       $593.4872   $593.49   $593.40   $593.50