When the OMs get together for our weekly game, we often debate whether or not to exchange a tile for an exposed Joker on another player’s rack. It’s a difficult decision especially if it is toward the end of the game when it could leave the player’s hand jokerless and then obviously worth more if she wins. Here’s some strategy instructions on this very issue from the Mah Jongg Madness newsletter:

Q.  I was told it was a good strategy NOT to exchange for a Joker if I didn’t need it, because it could give my opponent a joker-less Mah Jongg and I’d have to pay her more money.  Do you agree with this strategy?
Stephanie, IL
A. Generally, if you don’t need a Joker, it isn’t necessary to make an exchange.  For example – an opponent has exposed (2) 7Bams with a Joker 77J.  If you exchange that Joker with your 7Bam, then strategically…you are also preventing someone else (who may have another 7Bam in their hand) from getting that Joker.  You will also prevent the “exposer” from exchanging it herself, later.
     If your opponent’s exposures convey an obvious hand, i.e., 333 5555 77J (2013 card 11  333  5555  777  99) you know that all your opponent needs now is a pair of 1’s or a pair of 9’s to complete the hand for Mahj.  Your decision is now easier…”My hand’s not so good, so maybe I shouldn’t take the Joker – then I won’t make my opponent’s hand jokerless; and it won’t cost me an additional penalty.”   
     You are also (hopefully) aware of what other tiles your opponent has discarded from her hand – or how close she may be to getting Mahj.  (And for that matter…just how close you might be to making your own mah jong – if you only had that extra joker!)

     Again, strategies are situational. That’s why it is difficult to teach specific strategies in a Mah Jongg Theory class or to learn from a book.  Most times you have to be in an actual game in order to assess the logic of a specific strategy – and to make the strategic decision that works in the game at hand.