Creep is not a myth, it is arithmetic
Inner pages shift by the caliper of the paper, and the fore-edge shows the total.
Where the shift comes from
In a folded section, each inner leaf sits on top of the one outside it. The stack grows by one caliper per leaf, so the centre pages sit further from the fold than the cover. When the fore-edge is trimmed square, the inner pages lose more margin than the outer ones.
When it matters
A 16-page section on light stock may creep less than a millimetre. A 32-page section on a heavier sheet can move several millimetres, enough to make the margins look wrong on the centre spread. The number is predictable, which is the point.
The check to keep
Calculate creep from the actual caliper and page count, then apply the compensation in the imposition. Fold a proof and measure the fore-edge before the run. If the total is under 2 mm, record that it was checked rather than assuming it away.