2004-10-04 : If there are multiple attacks in a turn, then you have to pay for each attack.

2004-10-04 : If there is more than one Propaganda on the battlefield, the cost is cumulative.

2004-10-04 : Paying this cost is not an instant or any other kind of ability, it is an additional cost on the declaration of the attacker.

2004-10-04 : The payment is made during the declare attackers step at the same time you are declaring the attacker.

2007-02-01 : In the Two-Headed Giant format, you still only have to pay once per creature.

2014-02-01 : Unless some effect explicitly says otherwise, a creature that can't attack you can still attack a planeswalker you control.