It is the movement/rubbing in the washing that causes the most wear on the fabric. The purpose of dense quilting was always to hold the quilt together in the wash. There is nothing that breaks down a quilt faster than repeated machine washing -- if I had stored it away it would be just fine and nothing would have rotted.
Most of the sashing fabric is probably left over sheet trims. There are definitely different weights and weaves. In that case, the weave of the sheet weight holds up better than the cotton.
Due to the allergies, I've always had to use as mild and scent free of products as possible. We have very neutral water.