@JayWB: I also have studied electronics. While you may be technically right, when you’re dealing with 12 volts, and at the battery, not at the end of any one circuit, any difference in potential is going to be negligible at best. We’re not talking a pole transformer fed by a HV line here. Try it–you will get just as big a spark whatever connection you decide to make last. If you can tell the difference, you are blessed with far better perception than I have. For that matter, if you have a coil connected to a battery, it hardly matters which side you disconnect–the field will still collapse and you will get just as much of a surge from the stored energy. The only difference is which way the current will be flowing for the surge, and thus which direction the spark will spray. Having switches ground a circuit in an automobile may be slightly better for the longevity of the switch contacts (very slightly), but with all circuits ending at the battery terminals, it’s like trying to notice the flush of one toilet at the end of the sewer line, assuming accessories in the car have been left on.
As (hopefully) you know, the reason for making the last connection the negative one, is the vehicle’s chassis being connected to the negative pole of the battery, so there’s less danger of slipping and making a really impressive spark should the chassis be contacted with the positive cable, and ensuring that the last connection to be made is far from the battery.