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 $17.96   13.335%   $20.355   $20.36   $20.35   $20.40  
  Purchase #2 $35.44   11.709%   $39.5897   $39.59   $39.60   $39.60  
  Purchase #3 $176.59   7.244%   $189.3822   $189.38   $189.40   $189.40  
  Purchase #4 $18.44   12.378%   $20.7225   $20.72   $20.70   $20.70  
  Purchase #5 $113.37   8.423%   $122.9192   $122.92   $122.90   $122.90  
  Purchase #6 $77.30   5.421%   $81.4904   $81.49   $81.50   $81.50  
  Purchase #7 $110.66   3.727%   $114.7843   $114.78   $114.80   $114.80  
  7 purchase totals = $549.76       $589.2433   $589.24   $589.25   $589.30