I can tell you from 35+ years of emergency service (LEO and EMS) that I have never unbuckled a dead person. I'm sure someone will come back with the out -of-the-ordinary story where a seat belt did not stop the person from being killed like Dale Earnhardt Sr. However, in the hundreds of crashes I have responded to and the occupants were buckled in, they survived. I don't back out of my garage without being buckled in.