Elliot first (or, alternately, Corvin first) and Ewan last is all I usually recommend. It's definitely not required to play in this order by any means. And doing a different order won't spoil anything. It's just that Elliot kind of deals with Nora's story in the least amount of depth and Ewan's route goes into the most depth.
I've heard plenty of people say they played Ewan's first (or even kind of prefer Ewan first), so this recommendation is my *personal* recommendation. Not the official recommended order or anything!