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 $221.88   11.841%   $248.1528   $248.15   $248.15   $248.20  
  Purchase #2 $40.43   10.325%   $44.6044   $44.60   $44.60   $44.60  
  Purchase #3 $58.48   13.753%   $66.5228   $66.52   $66.50   $66.50  
  Purchase #4 $111.86   12.537%   $125.8839   $125.88   $125.90   $125.90  
  Purchase #5 $49.99   3.436%   $51.7077   $51.71   $51.70   $51.70  
  Purchase #6 $140.69   3.626%   $145.7914   $145.79   $145.80   $145.80  
  Purchase #7 $63.74   4.461%   $66.5834   $66.58   $66.60   $66.60  
  7 purchase totals = $687.07       $749.2464   $749.23   $749.25   $749.30