No, you absolutely do not want to use iDVD to backup videos you purchased via iTunes. Movies purchased via iTunes will be in the m4v (MPEG-4) format, which is very highly compressed, and you also have the option of downloading HD movies via iTunes. iDVD will burn a video DVD disc. Video DVDs are SD resolution only, and must be in MPEG-2 format. So you will be taking a highly compressed video and compressing it again which is guaranteed to noticeably lower quality (plus loose resolution if you started in HD).
If you want to backup your iTunes purchases to disc, it is really simple. Launch iTunes. Under the File menu, select Library > Back Up to Disc. Follow the prompts and everything you purchased will be backed up to disc.
If you just want to back up a single movie to a disc, then click on the Finder. Under the File menu select New Burn Folder. Find your movie (its in your Music folder) and drag it into the Burn folder (it will actually stay where it is and automatically create an alias.) Double click on the Burn folder to open it up. In the upper right you will see a button labled "Burn". Click that and it will burn a disc for you. After you are done you can throw away the Burn folder, like I said the original file is still in your music folder.
