busy fingers - it's the opposite, she has 1/4" all around the foundation with the strings on it, and there is no foundation under the edges of the blocks.
Are the seams of the string fabrics sewn all the way to the edges of the 12.5" line? If not, you will may have open seams if you leave these at 12.5" unfinished. As far as adding a foundation under the sashing, I don't think it would be necessary. When a quilt is finished, there are always some areas (seams, applique, etc.) where there are more layers of fabric than in other areas.