Excellent question!
Ellen White herself was against belief in her gift being a requirement for membership, so no, you do not have to believe in Ellen White to be a Seventh-day Adventist.
On the other hand, most Adventists do believe in Ellen White. And whether you believe she was a prophet or not, her writings are good, uplifting reading, particularly her best known works: The Desire of Ages, Steps to Christ, and The Great Controversy.