I have heard arguments for both sides in regards to laying posters on each other (or stacking folded posters together, whatever) or separating.
Right now, mine are folded and in a box, and the rolled ones are in bags and in tubes (I know, I know).
My flat file has mostly prints that are in sleeves (poly and mylar) as well as flat movie paper (one sheets, daybills, inserts, half sheets, the works).
I agree with Rich that the sliding from using any sort of sleeves can be an issue, that needs to be controlled.
Also, Dale's point is a good one in terms of keeping a poster with stains away or not in direct contact with others. Also consider markings such as stamps (?) or marker, etc. that is usually found on the back of older one sheets. Just something to think about.
I would also suggest using acid-free paper such as Japanese mulberry paper that you can cut just bigger than the posters. This way the sliding is not an issue (or as much), and the posters are protected obviously.