I started using a 1/2 cup of vinegar for my softener with my laundry. Whether quilting fabric or clothing I put 1/2 cup white vinegar in the fabric softener part of machine and let it do what the downy would do. I remember having to put it in the final rinse water for cloth diapers so it would cut the detergent and soften the fabric. What I have found is that it also helps set the colors in the fabric. I always spray starch my fabric before cutting whether prewashed or not. When a quilt is finished I wash it the same way with the vinegar and it feels softer to me. I also have allergies and it isn't very much fun trying to shop in a quilt store or Fabric shop, especially when a new shipment of fabrics come in.
Good Luck
Suzy