Interesting thread - I recently replaced a tube on a light twin and didn't notice any 'creep marks' but will have another look. I would have thought that at 40psi there would not be any creep to speak of, perhaps with minimal inflation one might expect it (I well recall tyre clamps to prevent it on some of my early motorbikes!).
Also have seen a number of flat-spotted tyres where people have landed with brakes on (ouch!) or run off the end of the runway, certainly no ripped valves in any of these cases so if the tyres did creep then all that baby powder did its job!