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 $60.38   6.624%   $64.3796   $64.38   $64.40   $64.40  
  Purchase #2 $115.71   9.595%   $126.8124   $126.81   $126.80   $126.80  
  Purchase #3 $47.44   7.139%   $50.8267   $50.83   $50.85   $50.80  
  Purchase #4 $222.92   4.573%   $233.1141   $233.11   $233.10   $233.10  
  Purchase #5 $56.69   6.772%   $60.529   $60.53   $60.55   $60.50  
  Purchase #6 $41.82   6.600%   $44.5801   $44.58   $44.60   $44.60  
  Purchase #7 $13.49   3.579%   $13.9728   $13.97   $13.95   $14.00  
  7 purchase totals = $558.45       $594.2147   $594.21   $594.25   $594.20