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 $121.65   6.810%   $129.9344   $129.93   $129.95   $129.90  
  Purchase #2 $18.50   10.831%   $20.5037   $20.50   $20.50   $20.50  
  Purchase #3 $68.97   3.813%   $71.5998   $71.60   $71.60   $71.60  
  Purchase #4 $134.28   7.359%   $144.1617   $144.16   $144.15   $144.20  
  Purchase #5 $13.96   8.124%   $15.0941   $15.09   $15.10   $15.10  
  Purchase #6 $185.83   3.420%   $192.1854   $192.19   $192.20   $192.20  
  Purchase #7 $154.35   6.358%   $164.1636   $164.16   $164.15   $164.20  
  7 purchase totals = $697.54       $737.6427   $737.63   $737.65   $737.70