[Leon]

I almost always preach not to restore.....but on the poster, for me, I am not so sure. There are several factors to consider. What is the value of the poster in as-is condition vs preservation and/or restoring (2 different things)? Sometimes restoring can greatly diminish value. However, for a poster which isn't too valuable, and would display nicer without the stain, I would consider it. If I have piece that is made to be displayed I would always, at least, consider it. My Horner Composite has a tiny, tiny bit of restoration...probably less than 2 inches on the entire piece.....while I am not the one who had it done I would do it again if it were left to me. It was also deacidified and lightly cleaned for preservation (*by Graphic Conservation in Chicago)......
It comes down to what looks better to the owner when viewing it, if the value is the same. I would strongly consider getting the stain out if it were mine (and after I determined it wouldn't hurt value).