That's why it really depends on how you look at your lost. Going back to 15% lose, I would say that its not a good thing to sell because we all know that the market is very volatile and a swing of 15% regain is very common. So it really depends on a lot of factor specially how you look at your investments, whether short or long, just saying.
As the cryptocurrencies prices are extremely flexible, today you can lose 15-% but tomorrow you can be in a 20% plus. Thus, there is no one reason to sell your altcoins and Bitcoins.