I would echo Mark's reply exactly.
Yes, posters that have been degraded by
large tears, fragility, missing paper, tape & the like will have to be backed & restored. But folds as a defect? sorry.. NO!
Once folded a poster has been damaged and is vulnerable to fold separation
sure, but when? How many years or decades? I have a number of posters that are more than 100 years old and there is no fold separation at all, so as long as the posters have been cared for, such an issue is no problem.
my philosophy on backing or restoration is simple.. do as little of it as is possible and only when absolutely necessary. I collect paper, not linen.
I returned a lobby card to someone this week that had been gel-backed. It wasn't mentioned because the seller didn't consider gel-backing restoration (I'd like to know at what point he does consider it restoration)
Some people have said to me "museums linenback their posters".
yes, they do occasionally, but it's rare, for a variety of reasons, the main one being 'money'. Museums are not going to waste valuable funds that could be directed to more important projects, moreover - unless they want to exhibit the item, they don't do anything to a degraded item other than to place it between mylar sheets to arrest any further degradation. Even when backing an item that is missing paper, for the most part, museums do not restore missing sections - even for exhibit - of many items (particularly documents).
IMO, linenbacking and restoration is a vastly over-used method done too often simply for cosmetic reasons
but there is another factor... the financial issue
if a poster is worth $500 in it's current condition, and after getting $200 worth of backing & restoration it is now a $700 poster, what did you gain? Cosmetics? Big deal!
as a dealer, that math is even worse. If I spend $300 to acquire a poster that I sell for $500 (presuming there is no price compromise at sale), my profit is 66.6% of my cost.
If I linenback the same poster for $200 and sell it for $700, my profit margin has been reduced to 40%.. Where is my benefit? It looks to me that I just took a hit and made nothing extra, while I can otherwise take that same $200 and buy a poster I'm going to sell for $350. So for that same $500 in costs, I now have 2 posters worth $850.00 rather than one worth $700.00
we.collect.paper in this hobby.
paper