I've rolled the tops on my classic panniers many times in hot and cold temps and never noticed that they were hard to close nor had any issues with stress cracks. The older set of classic back rollers I gave to a friend are going strong at 10years+. That's why I didn't mention it - I haven't had a problem to date.
Hey Vik, we just got our new classics (and after a couple training rides are very happy with them). Since you've been using the classics for such a long time would you recomend closing them by making a loop over the top or using the shoulder strap. So far we've found it much simpler to just roll them up and make a loop over the top than having to deal with the strap.