No, you dont need to sell all altcoins. If you believe in its potential then keep and hold it. If you see another coin being more profitable just sell it and buy that one at the right time to get the best bang for your buck. If you want you can always partially hold your investment and sell another part for something else and then you dont have all your eggs in one basket. Its honestly up to you and how you view what you invest in and what direction you, again, see it going.