There are many explanations to your simple question. If you think the price is falling, don't sell it whether it is altcoin or even a bitcoin just keep holding on whatever coins is that. You are going to wait at some point that you are saying it is coming back. Many people do are panic buying and panic selling. I would suggest to you the best. Just buy if the price is falling and sell if the price is rising again.
It's worthy to keep bitcoins if price is falling, it will recover sooner or later, and gain some more value, but some coins have straight way down and its not smart to keep any altcoin. What to do when prices are falling is not so simple question, many factor's needs to be taken in consideration if you wish to give full answer, is it better to sell and cut loses, buy more at the dip or just wait price to recover is something that depends from which coin you choose, its simple with bitcoin buy more if you have or just wait but with others coins you should think couple times before you do something. Its more easier to work with more informations, and its easier to give better advice's.