Sorry to hear of your accident - always a stressful occurance! I too have been rear-ended quite badly; my car was a write off and 24 years later I still have shoulder and neck problems. One thing with whiplash is that it takes at least 3 days for the onset of REAL pain to begin, with it topping out about 7 - 10 days and then the pain stays for quite a while unless you seek out treatment via massage therapy or physiotherapy. I would at least get to the doctor and advise him/her that you've been involved in an MVA, that way its at least documented medically in the event you need more specialized care down the road.
Take care of yourself and if it hurts, don't downplay it..get it treated. It will be covered on the other guys insurance.