I'm sure Dr Harry Caul could make a graph, but new (after the early 80's ) posters have to be mint/perfect condition, but as you go back in time, you can forgive more problems. For very scarce posters, even more forgiveness maybe be needed. So writing on the back, as said, as long as there is no bleed through all is well with the world. In fact I think it adds to the poster, it's all part of the poster, and sometimes Cinemas history...