Joker Exchanges Are Optional (Article ID 62)
Abstract: There is no requirement to do a Joker exchange if you have a tile that would allow you to do so. In some cases, a player may choose not to do a Joker exchange if they want to prevent another player from being jokerless or to go for a jokerless hand. Taking a Joker to prevent another player from getting it should be considered.
Question from a player: If I have a symbol tile that can be exchanged for a joker, am I obligated to do the exchange?
Answer from the NMJL: “Joker or Jokers may be replaced in any exposure with a like symbol tile or tiles by any player, whether picked from the wall or in a player’s hand, when it is the player’s turn, however, you do not have to exchange a Joker.”
Source: Mah Jongg Made Easy 2020 page 22 #3
Other answers from the NMJL: “A symbol tile must not necessarily be exchanged for a joker if player wishes to try to go mah jongg without a joker or does not wish to take the chance of their opponent going mah jongg without a joker.”
Source for other answers: National Mah Jongg League Bulletin Q&A (1969 page 4, 1971 page 5, 1998) https://www.nationalmahjonggleague.org