With a friend of mine we were wondering exactly the same thing : Do we have to sell our altcoins for ethereum and Bitcoin ?.. My think is that the best is to sell 50% of all the little ALTCOINS I've got because in case it grows I would regret to have sell everything. We can separate different kind of ALTS : the really new one and the "old" ones who are in the top 100 of crypto. in the 100 top of crypto I invest in 10 and hold them for long term. Sometimes I sell 20% max  and put the money I get in Bitcoin. My purpose is to grow in BTC now.