We have sold many flags over the last 5 years (imported with finished print) on the 5'x3' size and there is give in the material when stretched out to full size. The larger the flag the more give and more chance of movement I would have thought. At the end of the day, you guys would have more experience than most by the sounds of it the the set-up you have. I have had the rippled type of blowout on large items under a flat press but I would have thought a rotary would avoid that. It just looks like a fold in the material as passing through but I guess you would see a crease as well then. I was about to start investigating buying a rotary but you are starting to put me off now. I know who to contact now though if I do get basic problems on one ;-)