For transporting large prints I've always had them rolled. I feel this is safer and is much more cost efficient if you are shipping via Fedex. As for long term storage, flat in an archival box. I've kept some of the paper boxes from the darkroom days that are quite large - they live up in the attic due to size.
Vintage, thanks, I have the 24x30 case but if I print 24x36, I guess I will roll. I see a benefit of rolling in decurling. I have been delivering larger prints between 2 pieces of cardboard taped so they can transport for matting and framing with out damage. Speaking of which, have you found curl is an issue with roll paper especially near the end of the roll? Can you get the curl out simply by leaving the print between 2 sheets of cardboard protected by acid free paper for a few days or is a decurler needed?