It is a Judgment call. You are not legally required to, and DOR will most likely not even bother to read it, but the HO or Judge may appreciate a 'road-map' of the defenses in advance. Make sure you are on sturdy legal grounds before putting anything in writing, however, because if you lose, DOR will come after you for fees and sanctions and will get both.
This information is a general answer and is not specific to any particular case. Carin Manders Constantine, Esq. 727-456-0032/ 727-488-8272 familylawyer411.com/about-carin https://www.facebook.com/pages/The-Law-Offices-of-Carin-M-Constantine/125967577416313 http://www.linkedin.com/pub/carin-constantine/b/861/445
Frankly, I don't think DOR will read anything you file. The office is just so busy. It would probably just be a waste of time. You can always state that other grounds will be argued ore tenus.
Legal disclaimer: Ms. Braaten's answer to your question does not establish an attorney client relationship, but rather is meant to share knowledge with the general public. For specific advise on your case, you need to consult one on one with an family law attorney.