Originally Posted by JCL

Not sure I understand your logic, as once the negative battery post is disconnected the positive is not alive, there is no complete circuit.

Assume a negative-ground vehicle (which is a pretty safe assumption, these days):

1) If you remove the positive cable first, while you are loosening the positive terminal your wrench may contact the chassis and draw a spark. Once the cable is off, you are clear.

2) If you remove the negative cable first, there is no danger if your wrench results in a short to ground, since the cable is grounded in any case. Once the cable is fully removed, you are clear.