Many ICOs have their token sold at a fixed price vs. the ETH. For example: 1 ETH = 1000 Tokens.
So does it really matter if the prices of ETH/BTC is dropping?
If anything, then buying now you get more Tokens for the $, right?
Personally, this year (2018) is not a good time to buy or join an ICO project.
But if you want, you can do research first before starting to invest in.