Most definitely, a very big reason. The ability of down to trap warmth and insulate is based off the feathers loft or "fluff". Storing down in a compressed form diminishes the ability of the feathers to reloft, thus lessening their heat trapping capacity. Basically, storing compressed will flatten the feathers and ruin your bag. Store either opened large in a cotton storage sack, or hanging up in the closet. Though don't drape over a hanger, as this can stress a certain portion of the bag. Hope that helps.
Eric