I agree, "natural stance" seems to make the most sense.
I generally assume my waist to be my narrowest part (which I hope to make narrower still!
) of my midsection. For me, that's an inch or two above my navel, but every body is a little different there.
The most important thing is consistency. If the tape winds up slipping to your natural thinnest part, that is your waist; if you've got a more athletic build or are an "apple" shape still working on finding your core, then go with a landmark like your navel or a beauty mark or something so you can get consistent measurements of the same area.