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 $107.75   4.125%   $112.1947   $112.19   $112.20   $112.20  
  Purchase #2 $14.55   5.803%   $15.3943   $15.39   $15.40   $15.40  
  Purchase #3 $20.64   3.525%   $21.3676   $21.37   $21.35   $21.40  
  Purchase #4 $227.41   13.274%   $257.5964   $257.60   $257.60   $257.60  
  Purchase #5 $67.71   3.656%   $70.1855   $70.19   $70.20   $70.20  
  Purchase #6 $52.99   8.606%   $57.5503   $57.55   $57.55   $57.60  
  Purchase #7 $48.90   13.757%   $55.6272   $55.63   $55.65   $55.60  
  7 purchase totals = $539.95       $589.9160   $589.92   $589.95   $590.00