It was my understanding that setting the book type only applied to DVD+R media (and not to DVD-R media). I assume you are using DVD+R media.
I am a Nero and DVD+R media user. I allows create DVD with book type = DVD-ROM and have first hand experience that it does make a real difference with player compatibility (e.g., with older DVD players).
Whether you can control the book type on a DVD is also a function of your DVD writer. Some allow user control (via a special applications or via burning programs like Nero), some do not, and some simply default to DVD-ROM booktypes. I'm not sure the book type control situation is with your particular DVD writer. If you are not sure visit http://www.cdrlabs.com/ where their reviews typical discuss book type control.