I honestly can't believe that the direction the brush stands drying would have much influence on the longevity of the brush. There are enough glues available that are totally unaffected by moisture, and I can't imagine the manufacturers not using a waterproof glue. I also think that the entire brush gets wet during use, due to capillarity. I have allowed my boars hair brush drying standing up as much as hanging down in a stand, because I wondered which way it dried soonest. Never noticed any real difference.
Some sources mention that swirling and twisting motions during shaving and drying should be avoided, because such actions can cause the hairs to become loose. Those sources advice to only use painting style motions. I personally do some careful twirling while massaging the lather into my beard. My boars brush sheds one hair every dozen shaves or so, my badger (a fabulous Semogue Silvertip) hasn't lost a single hair so far, but it has only about 6 or 7 shaves on the counter.
Grabbing the brush beyond the handle, supporting the hairs a bit more while twirling, IMHO would make more difference than the storage direction.
Kind regards,
Bart.