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 $239.71   11.494%   $267.2623   $267.26   $267.25   $267.30  
  Purchase #2 $72.87   3.682%   $75.5531   $75.55   $75.55   $75.60  
  Purchase #3 $172.45   3.692%   $178.8169   $178.82   $178.80   $178.80  
  Purchase #4 $236.84   10.388%   $261.4429   $261.44   $261.45   $261.40  
  Purchase #5 $79.71   8.274%   $86.3052   $86.31   $86.30   $86.30  
  Purchase #6 $172.71   4.759%   $180.9293   $180.93   $180.95   $180.90  
  Purchase #7 $205.71   7.724%   $221.599   $221.60   $221.60   $221.60  
  7 purchase totals = $1,180.00       $1,271.9087   $1,271.91   $1,271.90   $1,271.90