It all depends on what kind of cryptocurrency you keep. If it is Bitcoin, Ethereum or other top Altcoins, I will advise you to keep them for a long time. And if these are coins of another project that nobody needs, then of course sell them.
Yes, I think it's a good idea to invest with popular coin that leads in higher value and hodl for a long time in able to be secured to face future ,and also to invest ot buy with other potential coin I think it is,necessary for us to sell and buy in able to earn small profit at the same time or shall we participate in campaigns to earn rewards.