It depends on what type of files were backed up for restoration.
If data was backed up or being backed up due to malware infection, the safest practice is not to backup
any files with the following file extensions: exe, .scr, .ini, .htm, .html, .php, .asp, .xml, .zip, .rar, .cab as they may be infected if your machine is infected. If you attempt to restore such files to a clean machine via a CD or usb drive, they could result in spreading infection.
As for backing up to a usb vs CD/DVD...the point is not so much that CD/DVD's are a better option but a safer one. Why...because most malware writers take advantage of the Autorun.inf
that uses Windows Explorer's right-click context menu for usb drives and external hardware devices. They do this because folks use them more often these days to back up data since external hardware devices are more convenient and portable. Thus, infections are spread easier and faster by attckers exploiting that method so they take advantage of it.