At the risk of getting my own head blown off, I'll jump back in here to
clarify the point I was trying (not very well) to make.
The risk of explosion results when you make the last of the four connections
to the battery, rather than to a point away from the battery. The gases, if
any, are going to gather at and just above the battery. If you fail to make a
good connection on the first try, you may just create sparks. These sparks
would, in turn, ignite the gas above the battery, thereby blowing your hand,
and anything else nearby (like your head if you were leaning in to it), off.
Therefore, what I was recommending is exactly what the triple-A manuals, and
the battery cable manuals recommend. Do not attach the last of the four
connections directly to the battery.
Hope that is clearer (I'll leave the polarity variation of this question to
the physicists in the group.)