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 $27.70   9.447%   $30.3168   $30.32   $30.30   $30.30  
  Purchase #2 $16.39   13.177%   $18.5497   $18.55   $18.55   $18.50  
  Purchase #3 $178.96   3.559%   $185.3292   $185.33   $185.35   $185.30  
  Purchase #4 $193.78   5.849%   $205.1142   $205.11   $205.10   $205.10  
  Purchase #5 $107.99   4.491%   $112.8398   $112.84   $112.85   $112.80  
  Purchase #6 $230.72   13.246%   $261.2812   $261.28   $261.30   $261.30  
  Purchase #7 $71.44   10.346%   $78.8312   $78.83   $78.85   $78.80  
  7 purchase totals = $826.98       $892.2621   $892.26   $892.30   $892.10