My understanding: manufacturers recommend not connecting to the negative battery post simply to avoid the potential of an explosion (of the gases possibly emanating from the battery) if there is a spark.
Chances of an explosion are surely extremely slim for a battery in good condition.
On my Prado, there is a chassis grounding point 20 cm or so from the battery that I use. But I wouldn't have a problem using the post directly -- the risk is so low.
''Possibly'' ... remove that, gases do
eminate from lead acid batteries, by the nature of the chemical reaction Hydrogen is produced.
''Chances of an explosion are surely extremely slim for a battery in good condition.'' ... chances of an explosion are not reliant on ''condition'' .... chances are reliant on if a spark ignites an explosive mixture of Hydrogen and Oxygen, a battery will explode, even if brand new, as all produce Hydrogen.
Why I felt a cautionary diatribe? The nephew lost his right eye, most sensation in the right side of his mouth to a battery explosion. Yes when they go off it's with a bang better than a gun shot.
I am guilty of the odd 3 or 7 spanner sparks round a battery, I don't have a problem with going straight to the posts either. Lady Luck or the gas ratios hath been kind to me thus far, you only need see someone who has had it happen to respect that seemingly harmless plastic bomb that may never explode in a million years, or if the planets align it might be tomorrow.