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..
That is why it is said that as soon as the ICO is over first they should distribute the coins to the ICO investors and after 2 or 3 days of the coin getting listed in exchange other bounty coins should be distributed like this who ever have invested in ICO can get the movement of coin in initial stage of listing and some profit they can earn it. After that it is clear that the price will go down surely that time they can buy back the coins and keep for long term as per the project worth of that coin