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 $220.62   8.773%   $239.975   $239.98   $240.00   $240.00  
  Purchase #2 $207.63   3.399%   $214.6873   $214.69   $214.70   $214.70  
  Purchase #3 $97.90   11.686%   $109.3406   $109.34   $109.35   $109.30  
  Purchase #4 $155.31   11.822%   $173.6707   $173.67   $173.65   $173.70  
  Purchase #5 $128.91   9.758%   $141.489   $141.49   $141.50   $141.50  
  Purchase #6 $9.31   10.857%   $10.3208   $10.32   $10.30   $10.30  
  Purchase #7 $60.71   11.181%   $67.498   $67.50   $67.50   $67.50  
  7 purchase totals = $880.39       $956.9814   $956.99   $957.00   $957.00