Jim,
General rule is "Negative off first and on last".
Principle is simple. Swinging a wrench and connecting negative to structure is harmless; doesn't matter if the positive is connected because you did not create a circuit. Swinging a wrench and connecting positive to structure will arc if the negative is connected because the structure is grounded. So, remove the negative first to un-ground the structure. Connect it last because doing so is harmless.
None of this will save you if you swing a wrench against the other battery terminal. In the car business we buy or make a wrench with an insulated handle because people sometimes do dumb things. The other trick is to hand out wrenches with handles too short to reach the other terminal, but insulated is better.