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 $109.62   13.373%   $124.2795   $124.28   $124.30   $124.30  
  Purchase #2 $154.54   11.583%   $172.4404   $172.44   $172.45   $172.40  
  Purchase #3 $136.32   12.429%   $153.2632   $153.26   $153.25   $153.30  
  Purchase #4 $204.73   8.734%   $222.6111   $222.61   $222.60   $222.60  
  Purchase #5 $125.28   6.713%   $133.69   $133.69   $133.70   $133.70  
  Purchase #6 $54.32   8.430%   $58.8992   $58.90   $58.90   $58.90  
  Purchase #7 $96.58   11.491%   $107.678   $107.68   $107.70   $107.70  
  7 purchase totals = $881.39       $972.8614   $972.86   $972.90   $972.90