I think you are correct in thinking it is the sashing, corner stones and borders that give cohesiveness to some scrappy quilts.
I know that was the case with the mystery train ride scrappy on this board that I did. My squares were all different colors, styles and vintages of fabrics. I chose a single rather neutral fabric for the sashing and a particular dark background calico for the "stars". Then as I added borders the first was a color that complemented the stars and the intended room. The second border I did piano key of many of the fabrics in the squares as well as a few additional fabrics. For the final border I'm wanting to do another dark background calico. Thought I had one that would work, but it has more pink than orange so not sure it will look like I want it to. Although there is quite a bit of pink in the squares, I'm wanting it to come out more gender neutral as it is for my guest bedroom and my boys (or my girl either) aren't really into pink flowers.
