I do think we put far too much importance on this. I have some posters from the 20's and 30's that, (before I got them) have just been lying around,or stuffed in drawers, with no concern for preservation; And these are in pretty good shape after going through the blitz, and such like.
Now we Poster collectors want to keep them in as good a condition as when we got them....! Fair enough, but lets just be sensible. I keep my posters in a room with the curtains shut, unless I'm in there. The central heating is never on in that room, and the door keep shut. So even in the summer, it doesn't get too hot in there.... And that's about it. Why would you want to do anymore...