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 $93.57   5.141%   $98.3804   $98.38   $98.40   $98.40  
  Purchase #2 $75.46   9.871%   $82.9087   $82.91   $82.90   $82.90  
  Purchase #3 $9.95   10.253%   $10.9702   $10.97   $10.95   $11.00  
  Purchase #4 $219.29   13.468%   $248.824   $248.82   $248.80   $248.80  
  Purchase #5 $200.89   12.601%   $226.2041   $226.20   $226.20   $226.20  
  Purchase #6 $75.70   4.287%   $78.9453   $78.95   $78.95   $78.90  
  Purchase #7 $107.34   8.604%   $116.5755   $116.58   $116.60   $116.60  
  7 purchase totals = $782.20       $862.8082   $862.81   $862.80   $862.80