I don't know if plastic wrap is acid free or not, (I doubt that it is), but I still think any plastic next to cotton in storage is a bad idea. That's because there will be condensation forming on the plastic with temperature fluctuations, and that moisture will be trapped by the plastic, causing mildew to the adjacent fabric. Quilts should never be stored long term in plastic bags for that reason, so I think the same logic follows for storage of unused fabric.