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 $24.68   11.260%   $27.459   $27.46   $27.45   $27.50  
  Purchase #2 $220.32   13.366%   $249.768   $249.77   $249.75   $249.80  
  Purchase #3 $143.43   9.309%   $156.7819   $156.78   $156.80   $156.80  
  Purchase #4 $233.71   6.825%   $249.6607   $249.66   $249.65   $249.70  
  Purchase #5 $132.89   3.232%   $137.185   $137.19   $137.20   $137.20  
  Purchase #6 $14.77   8.822%   $16.073   $16.07   $16.05   $16.10  
  Purchase #7 $24.62   11.433%   $27.4348   $27.43   $27.45   $27.40  
  7 purchase totals = $794.42       $864.3624   $864.36   $864.35   $864.50