Hmm OS, have you ever accidentally grounded yourself against the car? I certainly have and that 12 volts is enough to knock you on your butt. You can weld with a 12 volt battery so don't think you can't harm yourself...
And while it is true it is easier to hit ground while working on the positive side this is not why it is a bad idea. Since the electrons run out of the battery at the negative terminal you are likely to spark much more from the negative post than you are the positive post. But of course you are going to tell me my training and experience mean nothing or that somehow electons just don't flow the same where you are...
You are confusing a low voltage battery with the inductive kick given by loads. But that also happens on the +ve side (thanks to duality or symmetry or whatever it's called).
However I suspect you are also talking about holding a connector whilst disconnecting it - eg, the +ve or -ve battery terminal. You won't get a kick by connecting yourself in series with the battery - unless perhaps you have another power source present (another battery powering the circuits, or a welder, etc). But hold across connectors when (relay) coils de-energise etc and yes, you'll get a nice kick.
And welders work on current. What your saying is like saying that because a battery can melt a screwdriver, it can electrocute you or give you kick.
As to non-symmetry because "electrons from the -ve terminal", that is plain wrong (not to mention the crap about current flow being merely "electron movement").