From what I recall, OB-GYN was about 10-15% of the exam. I dunno if not doing the rotation would screw you or not. If it were me, I think it would b/c the stuff I learned on OB was so different from the other rotations.
I would highly recommend doing the OB-Gyn rotation if you can. If there is one core you can go without it is Psych! The OB-Gyn experience will make it easy on you in terms of questions involving management!