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 $24.32   8.142%   $26.3001   $26.30   $26.30   $26.30  
  Purchase #2 $239.89   4.212%   $249.9942   $249.99   $250.00   $250.00  
  Purchase #3 $31.59   11.738%   $35.298   $35.30   $35.30   $35.30  
  Purchase #4 $9.83   10.717%   $10.8835   $10.88   $10.90   $10.90  
  Purchase #5 $17.83   11.798%   $19.9336   $19.93   $19.95   $19.90  
  Purchase #6 $223.87   5.827%   $236.9149   $236.91   $236.90   $236.90  
  Purchase #7 $24.48   4.566%   $25.5978   $25.60   $25.60   $25.60  
  7 purchase totals = $571.81       $604.9221   $604.91   $604.95   $604.90