No you don’t! If there’s a book too bad, too boring, or read too long ago, you don’t have to let it keep you from getting to the rest of your to-be-reviewed list. MsWas calls this “review amnesty,” and it is 100% ok. You might be a completist and feel the need to write something about every book, but that is not required for CBR.
If you want the book to count towards your total on the Leaderboard, however, yes—you must post a review that’s at least 250 words long. (150 words per book if a multiple book post.)