My gut feeling about seatbelts is tgat they are only fully effective if the person behind it is sitting properly.
So my sluochy DC are reminded constantly to sit up straight in their seats. For safety reasons naturally, not because it makes the car look tidier. 
AIBU to be particular that the seatbelts are positioned cirrectly or am I just a finnicky nag about sonething that is statistically unlikelt to be relevant?