When it is falling, a lot of people will ask you to buy or to hold, but you should look for the reasons why the price is falling, if its caused by usual correction then holding or buying is already correct, but if you found something suspicious, like the coin developer want to scammed people or there are weakness in the coin, then you should sell, investing in alt coin need patience and need a strong mind
There are many people regret for selling coin when its price goes down immediately. Actually, its very normal, because sometimes selling can help them reduce the risks. Nevertheless, as far as we know that the crypto currencies market is extremely volatile, it fluctuates currently, thus you should know that if you have plan for investing coins in long-term with the big profits, you should keep calm and patient because if it falls, it also increases. Youd better hold it, that helps you earn a lots.