if the ICO's project done and already listed in any exchange, sell it at once. Most of the ICO project are going to fall the total allocation of the token that said on the project.
It commonly dump because the real product ( token or apps ) is under the process of development. It usually dump because of those bounty hunter who want a real money than altcoins reward. Who the hell investor will sell those tokens in a low value if they buy it on a high value?
That is the main reason why some investors are not willing to invest again, because of the sudden dump. So try to hold those coins in a long term or at least a couple of months until the real product launch.