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 $11.64   7.326%   $12.4927   $12.49   $12.50   $12.50  
  Purchase #2 $87.62   12.772%   $98.8108   $98.81   $98.80   $98.80  
  Purchase #3 $221.75   8.514%   $240.6298   $240.63   $240.65   $240.60  
  Purchase #4 $182.28   6.791%   $194.6586   $194.66   $194.65   $194.70  
  Purchase #5 $20.38   11.670%   $22.7583   $22.76   $22.75   $22.80  
  Purchase #6 $28.71   7.744%   $30.9333   $30.93   $30.95   $30.90  
  Purchase #7 $115.92   11.797%   $129.5951   $129.60   $129.60   $129.60  
  7 purchase totals = $668.30       $729.8786   $729.88   $729.90   $729.90