What the insurance companies are looking for is an objective statement that you have a deviated septum and not just looking to get a rhinoplasty. A CT report does just that. MRI is not as accurate or specific for septal anatomy. If the septum is fairly crooked, some will accept photos taken of the inside of your nose with an endoscopic camera.
Web reference: http://www.mdface.com/proc_rhinoplasty.html