Hm. I feel like you should always disclose paid endorsements, even for things you would have endorsed for free. My favorite gaming news site, Rock Paper Shotgun, typically has a disclosure line if the writer knows or has worked with the game creator, even if they would have reviewed and enjoyed the game regardless. Even an honest reviewer can be unconsciously swayed--that's why "free stuff" for reviewers (food, event tickets, crazy press events) is such an ethics issue.