Nowsaday, I don't think there is any ICO worth to invest anymore. I had so many bad ones lately. Better off purchase them after they released at the exchanges.
What you are thinking is the correct thing because what ever the ICO worth is when they get listed and as soon as the bounty coins are distributed you can surely expect the price to be dumped and you can get below ICO price for sure as i have seen in almost all coins even if the project is good
Some of the dumpers are those who participated in the marketing campaign,they leap from ico to ico,get their bounty as payment and dump their coin at it hit the first exchange and because of this,the price goes down the drain,leaving the ico investors with a much lower price than they've paid for..