I have recently decided that I would not buy linenbacked posters anymore. One exception is this one of a kind poster that I know I will never find folded and belongs in my collection. For example, I have everything I want on Bardot, save for 2 or 3 pieces I have only seen once in 20 years of collecting. Should I encounter those pieces, I will immediately buy them regardless of condition and backing.
My reasoning?
1. I do not display my posters, so even for the large pieces mentioned by Antoine and Mark, linenbacking is irrelevant to me.
2. I do not trust linenbacking, I do not trust what has been done to a poster, the amount of work done to make it look like new.
3. Linenbacked posters take a lot of room, linen is thick, it cannot be stored in flat files and 5 posters fill up a tube. Since i have a fairly large collection, it would not work.
4. It allows me to go thru upcoming auctions very quickly and only check the folded stuff since 80% of any offering is linenbacked.
5. I genuinely like folds on posters, it gives it character, like wrinkles on an old man's face.
Now to answer Bigmike question specifically, I would first make sure the "ultra rare" unnamed poster is indeed ultra rare. Ultra rare to me means there are less than 10 copies in the world. If this is true and you want to display such ultra rare poster that you will never ever be able to upgrade, then yes, I would linenback it.
T