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 $244.93   13.631%   $278.3164   $278.32   $278.30   $278.30  
  Purchase #2 $208.78   3.422%   $215.9245   $215.92   $215.90   $215.90  
  Purchase #3 $56.82   4.406%   $59.3235   $59.32   $59.30   $59.30  
  Purchase #4 $98.67   9.445%   $107.9894   $107.99   $108.00   $108.00  
  Purchase #5 $26.72   13.220%   $30.2524   $30.25   $30.25   $30.30  
  Purchase #6 $43.44   8.482%   $47.1246   $47.12   $47.10   $47.10  
  Purchase #7 $148.71   7.259%   $159.5049   $159.50   $159.50   $159.50  
  7 purchase totals = $828.07       $898.4357   $898.42   $898.35   $898.40