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 $109.39   6.491%   $116.4905   $116.49   $116.50   $116.50  
  Purchase #2 $101.34   13.506%   $115.027   $115.03   $115.05   $115.00  
  Purchase #3 $28.88   6.736%   $30.8254   $30.83   $30.85   $30.80  
  Purchase #4 $10.86   5.265%   $11.4318   $11.43   $11.45   $11.40  
  Purchase #5 $198.54   6.554%   $211.5523   $211.55   $211.55   $211.60  
  Purchase #6 $121.57   6.402%   $129.3529   $129.35   $129.35   $129.40  
  Purchase #7 $11.73   11.632%   $13.0944   $13.09   $13.10   $13.10  
  7 purchase totals = $582.31       $627.7743   $627.77   $627.85   $627.80