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 $229.90   4.551%   $240.3627   $240.36   $240.35   $240.40  
  Purchase #2 $66.54   5.704%   $70.3354   $70.34   $70.35   $70.30  
  Purchase #3 $140.34   10.232%   $154.6996   $154.70   $154.70   $154.70  
  Purchase #4 $134.66   12.509%   $151.5046   $151.50   $151.50   $151.50  
  Purchase #5 $20.57   3.246%   $21.2377   $21.24   $21.25   $21.20  
  Purchase #6 $44.93   10.404%   $49.6045   $49.60   $49.60   $49.60  
  Purchase #7 $109.70   13.696%   $124.7245   $124.72   $124.70   $124.70  
  7 purchase totals = $746.64       $812.4690   $812.46   $812.45   $812.40