I’ve always just treated it as a natural 3D extension of the 2D grid rules

I believe that's how it's handled in D&D too, or at least how my table has always done it. I meant more as a practical matter, you're very unlikely to have a vertical wall grid and some kind of stand of the correct height for your minis, so you can't just count squares like you would for horizontal movement. That's when the Pythagorean Theorem comes up in my experience.