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 $50.82   12.697%   $57.2726   $57.27   $57.25   $57.30  
  Purchase #2 $101.40   7.391%   $108.8945   $108.89   $108.90   $108.90  
  Purchase #3 $195.47   7.751%   $210.6209   $210.62   $210.60   $210.60  
  Purchase #4 $187.29   3.383%   $193.626   $193.63   $193.65   $193.60  
  Purchase #5 $119.61   5.129%   $125.7448   $125.74   $125.75   $125.70  
  Purchase #6 $103.66   6.628%   $110.5306   $110.53   $110.55   $110.50  
  Purchase #7 $159.96   12.773%   $180.3917   $180.39   $180.40   $180.40  
  7 purchase totals = $918.21       $987.0811   $987.07   $987.10   $987.00