Whatever terminal is being grounded, that's the terminal you should mess around with first.

3 decades ago, there were still some odd makes that used "positive ground" electrical systems, and for those cars, you need to disconnect the positive battery terminal first.

It's a matter of safety *and* convenience, like the guys above said. I say "convenience" because you can actually "go against the rule" and come out none the worse from it, if you're just carefull enough--but then, why bother?