Spinal immobilisation protocols vary by state, region and even medical control hospital here, but in general we are supposed to immobilise stab wounds to the trunk. The theory being not so much because they may have fallen, but that the knife itself may have done damage to the spinal cord. I only have experience in the southwest region of Connecticut, but we (as EMT-Bs) are allowed to rule out spinal injury on conscious patients by a quick assessment. I know this is not true for all regions/states.