Doubling down when the dealer shows a 9 can be a good strategy, depending on your hand total. If your hand is a 10 or 11, doubling down is often a favorable move because the chances of drawing a 10, face card, or Ace are high, which could give you a strong hand like 19 or 21. On the other hand, if your hand is a 9, doubling down is less common because the likelihood of improving your hand with a single card is lower. However, if you're using a card counting system and the count is favorable, it might still be worth considering. The decision ultimately depends on your hand total and whether you have any additional insight from card counting or strategy charts.