You are right the market is always volatile and the top ranked coins can also come down. Have a look at the coinmarketcap on 2016 and you'll see a lot of top coins went down and lost attention from the market.
Though its highly recommende at this moment with those coins because they have higher market cap but it all depends on the development and what that coin's purpose. There are few coins on the top that aren't worth risking your money.
Yeah, but the top ranks coin are lower risky investment. Because you can't believe with unknown coins investment. Marketcap ranks always changing when different potential coin coming in the market. But i am not always invest in top ranks coin, i have a lot different coin even which is not listed in coinmarketcap.