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 $16.55   13.397%   $18.7672   $18.77   $18.75   $18.80  
  Purchase #2 $30.93   11.626%   $34.5259   $34.53   $34.55   $34.50  
  Purchase #3 $131.34   3.656%   $136.1418   $136.14   $136.15   $136.10  
  Purchase #4 $12.51   10.414%   $13.8128   $13.81   $13.80   $13.80  
  Purchase #5 $219.47   13.735%   $249.6142   $249.61   $249.60   $249.60  
  Purchase #6 $63.77   10.725%   $70.6093   $70.61   $70.60   $70.60  
  Purchase #7 $24.48   12.540%   $27.5498   $27.55   $27.55   $27.50  
  7 purchase totals = $499.05       $551.0210   $551.02   $551.00   $550.90