Just my two cents, take it or leave it... at 16 years old I was rear ended by a Chevette while I was waiting to turn left across traffic. I was driving a little Mini truck at the time (Can't remember which). I do remember looking up in the rear view mirror and having just enough time to take my foot off the brake and hit the gas before she plowed into the steel step bumper I had.
The impact threw the truck about 30 feet forward and a bit sideways and bent the bumper down into what looked almost like a roll pan. The Chevette looked like it had been made of butter and everything was smashed back about 3 feet.
My head bounced off the back glass pretty badly, and gave me some serious headaches for some time. Luckily I was 16 and the brain had plenty of room to rattle around still
I will be getting some seats with headrests for the simple fact that it can happen and it is not that big a deal to get them. I'll be buying new seats anyway. It's like having insurance. By the time you wished you had them it will be too late to get them.