Hi Mia, For my "heirloom" quilts I normally buy coordinating fabric from the top of my quilt. On large quilts this can be very pricey, but how many heirloom quilts does a person make in a lifetime. As far as the 108 inch backing fabric is concerned, I've seen nice quality but also some pretty poor quality at high prices. A 108" backing fabric if it is priced right and the quality is good can save you money for a king or oversized queen. Muslin is always an option, but I like the back of my quilts to look pretty when turned back on the bed. There is no right or wrong, it's a personal decision. I personally wait until my quilt is pieced to make a decision on borders and backing fabrics, which means sometimes I have to search for what I really want and often have to improvise. Best wishes, Bubby