If after the ICO ends and listed on exchange the price of their token is drop, in my opinion it is reasonable considering that they have just been listed on the exchange and they are still in the early stage (not yet having a product). So just hold them until have a decent value. However, if the ICO has ends a long time ago but the price continues to drop it seems like you have to sell this token immediately. Because usually this is due to an internal problem that occurs on this platform, and if you are stubborn to keep holding it you will only get loss more.