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 $240.36   11.596%   $268.2321   $268.23   $268.25   $268.20  
  Purchase #2 $175.49   11.613%   $195.8697   $195.87   $195.85   $195.90  
  Purchase #3 $18.69   9.204%   $20.4102   $20.41   $20.40   $20.40  
  Purchase #4 $197.63   8.526%   $214.4799   $214.48   $214.50   $214.50  
  Purchase #5 $169.26   13.545%   $192.1863   $192.19   $192.20   $192.20  
  Purchase #6 $85.25   11.807%   $95.3155   $95.32   $95.30   $95.30  
  Purchase #7 $13.80   3.452%   $14.2764   $14.28   $14.30   $14.30  
  7 purchase totals = $900.48       $1,000.7701   $1,000.78   $1,000.80   $1,000.80