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 $237.82   6.105%   $252.3389   $252.34   $252.35   $252.30  
  Purchase #2 $24.39   10.803%   $27.0249   $27.02   $27.00   $27.00  
  Purchase #3 $88.75   4.546%   $92.7846   $92.78   $92.80   $92.80  
  Purchase #4 $240.34   4.245%   $250.5424   $250.54   $250.55   $250.50  
  Purchase #5 $53.46   9.409%   $58.4901   $58.49   $58.50   $58.50  
  Purchase #6 $240.71   8.715%   $261.6879   $261.69   $261.70   $261.70  
  Purchase #7 $49.34   3.102%   $50.8705   $50.87   $50.85   $50.90  
  7 purchase totals = $934.81       $993.7393   $993.73   $993.75   $993.70