has anybody

I believe that this originated with early engines, such as flathead Fords, that had fewer main bearings and were made of softer materials. A 2 main bearing 4 cyl or 3 main V8 crank might very well warp if stored on it's side for any length of time. A modern forged crank is probably less likely to. However why take a chance? I was always taught to store them vertically, preferably hanging and I see no reason to take any chances.
A /6 crank laid on it's side for storage might very well warp.