I will answer from the Judge's point of view.
Some judges want to see the back stitches the same size and evenness as the front.
Some will want the back stitches to penetrate evenly and consistently.
I have seen few quilt in my many many years of judging that the back was the same as the top. So I look for the penetration and consistency of the back stitch. I have found it difficult to make the back stitch the same as the top due to the way the needle goes thru all the layers. You can control the top better than the back.