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 $18.70   12.238%   $20.9885   $20.99   $21.00   $21.00  
  Purchase #2 $101.73   6.422%   $108.2631   $108.26   $108.25   $108.30  
  Purchase #3 $200.79   6.694%   $214.2309   $214.23   $214.25   $214.20  
  Purchase #4 $34.57   7.281%   $37.087   $37.09   $37.10   $37.10  
  Purchase #5 $59.75   13.235%   $67.6579   $67.66   $67.65   $67.70  
  Purchase #6 $159.77   11.365%   $177.9279   $177.93   $177.95   $177.90  
  Purchase #7 $14.41   7.786%   $15.532   $15.53   $15.55   $15.50  
  7 purchase totals = $589.72       $641.6873   $641.69   $641.75   $641.70