When displaying a wallhanging quilt it is best to keep it out of direct sunlight since the strong sun will fade your fabric over time. It is best to avoid washing.
Large quilts will need to be washed over time, but a good airing will freshen up your quilt. Dry cleaning is bad because it will leave harmful chemicals in the fibers.
A mild soap such as Ensure or Orvis Paste (a shampoo originally designed for livestock) will be mild enough. A good soak in the bathtub works well to soak and rinse. The agitation of a washing machine will ruin your quilt and separate the batting in between. After ringing out the quilt well, lay flat to dry (outdoors) but enclose the quilt with bedsheets to protect it from sun and dirt. When your quilt is almost dry you can put it in the dryer on a cool setting to fluff it up.