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 $11.99   8.685%   $13.0313   $13.03   $13.05   $13.00  
  Purchase #2 $172.34   9.765%   $189.169   $189.17   $189.15   $189.20  
  Purchase #3 $242.77   7.744%   $261.5701   $261.57   $261.55   $261.60  
  Purchase #4 $31.98   5.248%   $33.6583   $33.66   $33.65   $33.70  
  Purchase #5 $107.80   5.351%   $113.5684   $113.57   $113.55   $113.60  
  Purchase #6 $30.92   10.611%   $34.2009   $34.20   $34.20   $34.20  
  Purchase #7 $206.32   7.849%   $222.5141   $222.51   $222.50   $222.50  
  7 purchase totals = $804.12       $867.7121   $867.71   $867.65   $867.80