Personally, I am a firm believer that "less is more" when it comes to poster restoration. With that being said there are many pragmatic circumstances when a poster should be stabilized with the use of the linen backing process, or tape stains airbrushed or
stains beached out. In many cases, I try to preserve the look and feel of the original poster, especially the flaws. For example, we just linen backed a three sheet for the film The Happiest Millionaire. The two pieces were about a half inch out from lining up, the worst I have ever seen. We adjusted the title so it lined up, but left the borders exactly the way they were. I think it adds character to the piece and preserves the era with which it was printed.
Again, I do believe man made damage can and in most cases should be fixed, like writing in the borders, a mustache drawn on the image, staining, trimming, etc. Posters from Quebec are legendary for having the English title painted out and replaced in French. This should be fixed.
I really enjoy a poster that is as authentic as possible.
Cheers, Dan Miles, Backingtothefuture.