War Tax

• WWizards of the Coast2004-10-04
  Official Ruling
  You can choose X=0. This doesn’t have an effect most of the time, but it does mean that creatures can’t attack unless the controller chooses to pay the cost (a cost of zero is not automatically paid, so the controller can choose to not pay).
• WWizards of the Coast2004-10-04
  Official Ruling
  The ability only applies to attackers declared after it resolves. It will not add costs to any attackers already announced.
• WWizards of the Coast2007-02-01
  Official Ruling
  In the Two-Headed Giant format, you still only have to pay once per creature.

Rulings via Scryfall