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 $19.35   4.412%   $20.2037   $20.20   $20.20   $20.20  
  Purchase #2 $18.39   3.792%   $19.0873   $19.09   $19.10   $19.10  
  Purchase #3 $188.25   9.724%   $206.5554   $206.56   $206.55   $206.60  
  Purchase #4 $177.58   6.533%   $189.1813   $189.18   $189.20   $189.20  
  Purchase #5 $174.45   12.416%   $196.1097   $196.11   $196.10   $196.10  
  Purchase #6 $118.62   12.394%   $133.3218   $133.32   $133.30   $133.30  
  Purchase #7 $115.92   10.275%   $127.8308   $127.83   $127.85   $127.80  
  7 purchase totals = $812.56       $892.2900   $892.29   $892.30   $892.30