As EMT's/Medics our backs are one our most important tools!!
I've just come back to work post a nasty back injury and I'm far more choosy about which patients I carry to the vehicle now.
You have to consider a number of factors I suppose - Complaint, Status, Weight, Extrication Difficulties, Distance to Ambulance......
I can think of examples where I've seen patients who are relatively sick being walked to the vehicle due to their weight or hazards in their home......
I don't think I've ever seen a written procedure here in New Zealand (Kiwi please point it out if I've missed it!!!) but I think anyone who's been around for a while knows who needs carrying and who doesn't!!