Thank you. My K25 retourne is only 6 months old and I store her on her back with sangles fastened in a drawer. Sorry for asking but have any of the artisans provided insights as to why they recommend storing the bag that way? Not at all doubting their recommendation, just curious to know.
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.
Just sharing how my retourne looks like now after 7 years of storing it standing. Note that this particular bag of mine has travelled the world with me including the swiss alps. I dont baby this bag.
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?!