Limited Editions can have all kinds of value. Some open editions have great value, some not, but may have in the future, who knows? (movie, circus posters, Or even news papers for example)
Editions with numbers and signatures outside of the image area, should be treated as the artist or publisher intended. Signed in the image area treat as intended.
As a framer for several major publishers, I can tell you that there are, hard and fast rules as to proper framing of this form of art reproduction.(ask anyone who paid excess of $10,000 for their opinion)
You are the professional, and should have a standard that you operate under. Educate your customer.
If in doubt make a copy and have at it. Oh but what about copyright, what ever do I do?
PS, We treat almost everything as if it were going in a time capsule, to be removed sometime in the future in prime condition.
Also the original intent of the Limited Edition has great merit, to bad that some producers of this art form got greedy with its proposed value.