It's always nice to get a treasure like this.
Agree, trim to 12" ... will be much easier for the math when you go to do your sashings, than to be dealing with a pesky 1/8"!
However ... in trimming, watch that you still have the 1/4" seam allowance available, so you will not be stitching into the plates when you put it together. In that case, you might sooner leave a couple out of the quilt.
Sashings don't have to have cornerstones ..... you could do them totally in yellow.
Or perhaps, you might find a print that would be quite effective.