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 $131.70   6.663%   $140.4752   $140.48   $140.50   $140.50  
  Purchase #2 $77.27   7.763%   $83.2685   $83.27   $83.25   $83.30  
  Purchase #3 $75.40   9.491%   $82.5562   $82.56   $82.55   $82.60  
  Purchase #4 $148.64   5.673%   $157.0723   $157.07   $157.05   $157.10  
  Purchase #5 $63.88   6.589%   $68.0891   $68.09   $68.10   $68.10  
  Purchase #6 $73.64   13.739%   $83.7574   $83.76   $83.75   $83.80  
  Purchase #7 $21.94   6.735%   $23.4177   $23.42   $23.40   $23.40  
  7 purchase totals = $592.47       $638.6364   $638.65   $638.60   $638.80