It's generally safe to just invest in the top 2 currencies, Bitcoin and Ethereum. However, if you resist selling at any moment, there can be a risk: you don't know what will happen in the future: the price might crash to the bottom, or some new technologies might be replacing those 2 coins. Therefore, I think a better strategy would be slowly selling a portion of your position when the price rises so that whenever something bad happens, you have already cashed out part of your positions and be safe.