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 $15.99   12.775%   $18.0327   $18.03   $18.05   $18.00  
  Purchase #2 $53.70   13.496%   $60.9474   $60.95   $60.95   $60.90  
  Purchase #3 $12.81   5.140%   $13.4684   $13.47   $13.45   $13.50  
  Purchase #4 $196.61   10.458%   $217.1715   $217.17   $217.15   $217.20  
  Purchase #5 $7.97   9.571%   $8.7328   $8.73   $8.75   $8.70  
  Purchase #6 $125.93   4.509%   $131.6082   $131.61   $131.60   $131.60  
  Purchase #7 $148.86   3.597%   $154.2145   $154.21   $154.20   $154.20  
  7 purchase totals = $561.87       $604.1755   $604.17   $604.15   $604.10