I agree with Mark here, I too roll many posters in one, and store them Horizontally. As Mark says, there is more chance of damage if stored Vertical, to the edges. When even the minimum of 10 are rolled together they are very sturdy and won't flatten out, unless a great weight is on top of them. Though I also have other rolled posters on top of those, and there has been no deflection in the bottom ones in the last 5 years..
My main problem is these mostly Vinyl Banners, how to store them. I still have most in the original box's, but the space they take up is unreal. I do have quite a few loose ones rolled together, but some have "knobs" on to hang them by (Que the jokes) , which get in the way some-what when rolling them, and slightly distort the edge. Any ideas......Stop getting them!