It's a very safe approach to only choose and invest in altcoins among the top 10, but for an altcoin to be good it doesn't necessarily have to be among top 10 cryptocurrencies listed in a certain website or an exchange because some altcoins might be good but they might not have the numbers to be on the top yet but they might eventually make it there. So, one shouldn't avoid all other projects and cryptocurrencies and just focus on the top ones as they may miss a lot of good opportunities because of that.

However, an investor must do their due diligence about a project or a coin before they go ahead and invest in it because where there are a lot of projects with great potential, there is a high number of scams and shitcoins in the surroundings as well and one must dodge them all to become a successful trader/investor.