One thing I can tell you is this: Do not use a border that is shorter than the body of the quilt. If you do, the quilt inside the border will 'bowl' or 'cup' - that is, it will never lay flat.
How do I know? I bought an adorable childs quilt at a thrift store for $2. All the borders were between 1/4 and 1/2" too short for the quilt top. To fix it, I had to remove all the quilting, seperate the top, batting and bottom, take the borders off...then I discovered that the cornerstone blocks of the borders were cut between 1/4 and 1/2" too big. So I removed the cornerstones and re-sewed them to the borders, then put the borders back on the body of the quilt.
The borders were now long enough to fit the quilt, although the cornerstones were not matched to the corners of the quilt body. Then I re-sandwiched and re-quilted the quilt. Long story short, it was still cute and no longer 'bowled' in the middle.
I will never do that again, too much work for too little gain, although anything I do that teaches me something is worthwhile.