No, you will get dividends when Iconomi enables Dividends payouts regardless if you bought at ICO or not.  You can, right now buy ICN at Kraken or Liqui and these ICN at today's market price and those ICN qualifies to earn Dividends just as good as those that were bought at ICO.

*If you buy at kraken you will have to transfer to your own personal wallet as Kraken hasn't publicly said if they will be distributing dividends to its users.
*If you buy at Liqui you can keep it there as Liqui has announced that they will be paying out Dividends to its users.

*I would still opt to transfer the ICN to personal wallets because alot of things can happen when keeping coins at exchanges, unless you are not technologically proficient at keeping your own computer virus free and your own tokens safe, then keeping it at the exchange with 2FA enabled with Google Authenticator might be more beneficial for you.