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 $28.40   8.836%   $30.9094   $30.91   $30.90   $30.90  
  Purchase #2 $200.70   6.762%   $214.2713   $214.27   $214.25   $214.30  
  Purchase #3 $12.53   5.217%   $13.1837   $13.18   $13.20   $13.20  
  Purchase #4 $112.71   12.241%   $126.5068   $126.51   $126.50   $126.50  
  Purchase #5 $131.40   6.459%   $139.8871   $139.89   $139.90   $139.90  
  Purchase #6 $193.36   12.863%   $218.2319   $218.23   $218.25   $218.20  
  Purchase #7 $66.92   6.662%   $71.3782   $71.38   $71.40   $71.40  
  7 purchase totals = $746.02       $814.3684   $814.37   $814.40   $814.40