That's been debunked here in the LuLa forums several years ago.
http://forum.luminous-landscape.com/index.php?topic=73604.0
Not really sure if that "debunks" it, certainly Alan's opinion is useful, but it is just one person's limited experience and considering he was using a 3880 it most likely referred only to the tendency of sheets to curl, not about the stiffness and curl in roll paper.
I'm not saying one way or the other, but I just don't think that link really debunks the idea, and the previous post certainly shows someone with the opposite experience, as does the post preceding Alan's in that thread.
in 17x22 sheets, I've found Exhibition Fiber, Legacy Baryta, and Legacy Platine to all be pretty "flat" and remain that way after printing. The slight edge curl on the Baryta is perhaps slightly more than the Exhibition Fiber, but really insignificant. From the Roll, I find the two paper pretty much identical, plenty of curl, have to handle with care, and really must be mounted if you want to eliminate the curl. I haven't tried decurling, I don't have anything big enough for the large prints I make when printing from a roll.
I wish the Legacy Baryta came in 24x30 sheets like the EEF. I tend to print that size quite a bit and loved how flat the sheets stay in my portfolio box. I may have to continue to use EEF for that project.
I am curious about this. I don't like the surface of the Platine quite as much, but it's a really nice paper. I think I'll order a roll or two and see how the curl compares.