I think Backing boards, or sleeve's are great if you have enough space, which I don't. I store most of my posters in flat file's, and the one's with tape, marks, or anything I'm not sure of, I leave folded in smaller draws.
I do keep the room in darkness, unless I'm in there. There is no central heating turned on in that room, which keeps an ambient temperature. I do think we can go over the top, it's not a museum, I have collected for 35+ years and non of my posters are any worse since buying them, and it's only in the last 5 years I've thought about storing correctly. Yet some of these posters are 80+ years old and look fine.