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 $34.39   11.692%   $38.4109   $38.41   $38.40   $38.40  
  Purchase #2 $240.95   3.233%   $248.7399   $248.74   $248.75   $248.70  
  Purchase #3 $75.39   3.677%   $78.1621   $78.16   $78.15   $78.20  
  Purchase #4 $47.38   8.154%   $51.2434   $51.24   $51.25   $51.20  
  Purchase #5 $201.27   6.303%   $213.956   $213.96   $213.95   $214.00  
  Purchase #6 $74.91   3.574%   $77.5873   $77.59   $77.60   $77.60  
  Purchase #7 $195.94   11.419%   $218.3144   $218.31   $218.30   $218.30  
  7 purchase totals = $870.23       $926.4140   $926.41   $926.40   $926.40