Official forks can manage to get holders credited in exchanges so that is the safest. But if the fork is unofficial they may ask you to provide the private keys to your wallet (when you keep them in one off exchange - you don't hold private keys on exchange, the exchange is holding your money for you...). If you are asked to provide your private keys think very hard because essentially the person who has the keys owns the coins (or has the ability to transfer them to another wallet). I'm with a lot of my readers on this one, looks scammy so be careful.