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 $213.59   12.348%   $239.9641   $239.96   $239.95   $240.00  
  Purchase #2 $78.47   7.230%   $84.1434   $84.14   $84.15   $84.10  
  Purchase #3 $64.75   4.735%   $67.8159   $67.82   $67.80   $67.80  
  Purchase #4 $48.49   10.717%   $53.6867   $53.69   $53.70   $53.70  
  Purchase #5 $120.91   13.295%   $136.985   $136.99   $137.00   $137.00  
  Purchase #6 $112.63   9.803%   $123.6711   $123.67   $123.65   $123.70  
  Purchase #7 $249.55   4.668%   $261.199   $261.20   $261.20   $261.20  
  7 purchase totals = $888.39       $967.4652   $967.47   $967.45   $967.50