It depends on the situation. Holding is very helpful but not all the time. Sometimes you need to sell your coin cheaper than to its original price to avoid the big loss.
That is true and it all balls down to the coin you are holding and the situation at hand. Recently, knowing that a huge dump was happening before the BTG fork, I quickly moved all my funds to BTC which is what any smart trader should do anyway once you stay alert.
I did not regret the losses cause eventually I made some back and still waiting for more dips at the moment to get more for pumps later on.