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.
Yes exactly it totally depends upon the current situation that how you take it. sometimes you have to sell imediately to prevent from loss but sometime you have to be stay calm but in general when Prices are falling of course don't panic if the value got dumped. Don't let your emotion control your mind and brain, so that you're gonna put in the situation that you can become totally wreck. You just have to be believe on your coin and you have to be confident on iit for the long term to get benefit from it because up and downs are temperory.