I use to keep stand vertical against wall, but have problems with this.
It is humid here, so the moisture stays low to the ground and bottom half of staves stay moist, sometimes mold in case of bamboo.
Maybe its not a problem in place with low humidity, but from now I will dry wood horizontally.
This is mostly true if direct sunlight does not shine on whole stave.
As long as you have air flow, which I think is most important.