My own experience in business is that they don't seem to care if you aren't wearing it at your own seat, but I wouldn't try to walk in the aisle without one. I try to put it on when interacting with them during orders, etc., but sometimes they catch me by surprise and have never said a word.

Even on domestic business last year when things were raging a bit more, they never cared if you had it on in your seat, just if you got up to go to restroom.