Selling off your coins depends on the investment plans you have in place for the particular coin you intend to sell. if you feel like its time you sold off after making considerable profits then you could do just that as your plans have reached,. In the case of BTC however, id advice that you keep holding as long as possible, we all have seen what the coin is capable of. even if it continues to dip in price, BTC has a tendency to recover after a period of time, therefore trust in it and hold on