Obviously, if the position is
then it is best to duck the king, but I was more concerned about
or, better yet,
where the play of the jack or ten when declarer leads from the ace might convince declarer to play you for JT tight.
Of course, if declarer has the ten when you have Jx, playing the jack gives declarer an easy run in the suit, and that comes up much more frequently, so playing the J from Jx on the possibility of an intra-finesse is not going to be a percentage play, but playing the ten doesn't seem to cost much in the right situation.
Thoughts?