Of course, it is better to distribute your funds to a lot of good crypto-currencies from the top 20. This way you minimize the risks. In the end, you will have other cryptocurrencies as a safety net. And this is generally the right investment system, when investment is everywhere.
Yes it is not safe to invest your money only on one crypto currencies it is very risky if the market value of the coins goes then you cannot do anything except suffering the loss. so be vigilant and invest some of the amount in top 10 crypto currencies which will helps you in getting profit from anyone of the altcoins.