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 $19.83   10.249%   $21.8624   $21.86   $21.85   $21.90  
  Purchase #2 $78.63   6.688%   $83.8888   $83.89   $83.90   $83.90  
  Purchase #3 $79.84   11.169%   $88.7573   $88.76   $88.75   $88.80  
  Purchase #4 $7.51   9.178%   $8.1993   $8.20   $8.20   $8.20  
  Purchase #5 $18.66   5.290%   $19.6471   $19.65   $19.65   $19.60  
  Purchase #6 $163.31   3.728%   $169.3982   $169.40   $169.40   $169.40  
  Purchase #7 $234.87   10.551%   $259.6511   $259.65   $259.65   $259.70  
  7 purchase totals = $602.65       $651.4042   $651.41   $651.40   $651.50