Except of course some of the single most expensive one-sheets - the nicest two Mummy one-sheets are unbacked, the Bride teaser, London After Midnight, the Style B Black Cat, the original Style A Dracula, etc.
As always, if you don't have to - don't.
Exactly. IMO, posters shouldnt be backed either, jus because they have been folded (as issued from the printer), but are in otherwise outstanding condition. (But to each his own, as well a know).
My point was that IF a rare or compromised poster was in need of conservation prior to it being displayed, that any museum would take those steps to assure its survival and ability to be put on display.
Old world master paintings that have suffered (tiny or larger) areas of paint loss or flaking have been touched up by expert restorers. The same would be done to a poster, if need be.
But as you said, Sean, if NOT needed, don't do it. And Im sure museums follow this strict ideology, too.