Does FFG have a stance on sleeved cards in organized play? I love using the clear sleeves because I think the card backs have great art, but I heard a rumour that they are not tourney legal because people will mark cards.

Is this a hard and fast rule, or does it go by the store/tourney director?

FFG does not have an official stance. They neither require sleeves, nor do they have any limitations if you choose to use them.