As far as altcoin and any asset is concerned seling at any point is a personal decision but lets be sincere every investor  would want to sell when he or she is comfortable and satisfied with the profit have made so far, in other way round, some people invest in altcoin for diversification sake and once their expecation are not in accordance with the results seen, they pull out immediately irrespective of profit made, the thing is that, it also depends on the coin involved, we can't confidently say that we know the particular coin that will pump tomorrow, no matter how good we are in crypto business these pumps and dumps just happens and it takes perseverance and patients to keep holding altcoin, to me,  once there is a little pump in altcoin that I acquired I will pull out the profit and reinvest my capital to avoid total depreciation or lost of my asset as the case may be, the major reason why I will take  the aforementioned decision is because altcoins are not fully trusted although not all of them but dealings with altcoins should be cautiously and carefully done unless as an investor you are convinced beyond reasonable doubt.