I agree. Got rear-ended about 12 years ago and my car took no real damage at all, though the idiot who hit me destroyed the front end of his Escort. Got my car inspected next day and there were no problems, so I stupidly threw the guy's details away, ignored the jarring, and got on with my life.
A couple of days later my lower back spasmed and I had a few days off work. And then again a few weeks later. I've had trouble with it on-and-off ever since and some serious physio, though to be fair it doesn't really stop me because I won't let it. Though I pay the price in the end.
Point is, don't write off any injury in haste. Keep a journal of it if necessary and don't risk anything. Look after yourself mate.