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 $243.90   12.537%   $274.4777   $274.48   $274.50   $274.50  
  Purchase #2 $53.46   5.422%   $56.3586   $56.36   $56.35   $56.40  
  Purchase #3 $157.91   10.137%   $173.9173   $173.92   $173.90   $173.90  
  Purchase #4 $78.92   12.340%   $88.6587   $88.66   $88.65   $88.70  
  Purchase #5 $98.36   9.393%   $107.599   $107.60   $107.60   $107.60  
  Purchase #6 $241.77   3.257%   $249.6444   $249.64   $249.65   $249.60  
  Purchase #7 $44.91   7.395%   $48.2311   $48.23   $48.25   $48.20  
  7 purchase totals = $919.23       $998.8868   $998.89   $998.90   $998.90