Well, on the bag's back because of gravity; in the dust bag because of dust and light; sangles fastened because it keeps the shape; and with airflow because it's better for the leather. Again just what I was told, but I have bags that are many years old and all are in great shape.
I do recommend taking bags out and checking them regularly. Occasionally a back pocket can have a flaw that can make an indent, or something the bag is lying on, even rough paper, can make a mark.
No one is telling you not to store your bag standing. As I said in my post, YMMV. I personally think standing storage can cause/exacerbate that slight ripple in the piping, which some folks mind and some like.

So many people here seem to worry about their Kellys slouching, so I tend to angle my advice in that direction. We all know that in the atelier they are hung by the handle 🤷♀️
The smaller the bag, the less slouching, so K25 is way less likely to slouch than, say, K35.
Anyway, again, there are bag storage threads; let's get back to things that relate to Kelly 25 specifically?!