Well in most cases, what you do with airdrop or ICO tokens and when you do it should be determined by some factors.  Such as your analysis of the tokens potential, your finances (portfolio), market situation etc.  There is no simple answer, some tokens are at their weakest (cheapest) immediately after hitting exchange while some others will gain overtime.  Whatever the case, it is very important to DYOR about each project you are involved in, if you understand the project white paper and roadmap, you are in a better position to decide if or when to sell.