even if you think laminating is OK now, Ill wager if you continue collecting you would regret having it done.
Unless you buy a couple of the same item, laminate one and keep one as it should be (assuming these are cheap posters).
I have some posters that someone has laminated.
I can never sell them, I do stick them on my wall and its OK, but they have no value, but a couple of them I could sell easily for $100 or more (easily) if they weren't laminated.
as toi linen backing new posters, there is no point, it would be cheaper to just buy a nice example than have a damaged one backed. And especially fopr modern posters you will never gewt your money back on a linen backed example. (I know you don't ever want to sell, but who knows in the future, and no sense throwing money away).
Probably for everyone a laminated poster would be the same as a commercial reproduction, and even that would probably cost you less than the price of laminating.
a collection post 1992, I wouldn't back any of them, I would only laminate if you want to stick it to your wall, no frame, and your worried the cat might piss on it or something or you want to use as a table cloth or something (I have seen lobby cards laminated as place mats, its kinda cool).
regardless I wouldn't stress about sun and humidity too much, just buy a spare, if the first fades replace it.