I understand the safety concerns leading to grounding to the frame or engine for the car with a dead battery. From what I've read the dead battery is the one with a chance of venting hydrogen, but every source I've seen says to make the ground connection there last. If sparking at the last connection is the concern then why not ground the good battery last?
Fair warning, this is a guess. Vote appropriately...
General accepted order:
- Dead +
- Good +
- Good - (directly to the terminal)
- Bad - (to a good engine ground, but not the terminal)
Avoiding the last connection directly to the dead negative is to avoid sparks near the dead battery, but this also avoids sparks near both batteries since the good one is securely connected at this point.
Connecting Positive first is good because doing it the other way makes it easier to accidentally short out the battery.
Now, why not start with the Good battery positive? I think the reasoning is that you want the best possible connection to the Good battery, so you really want the alligator clips of the cables connected directly to the terminals.
To connect the positives first and end with a negative connection away from the battery, the "recommended" method is the only order that satisfies everything.