I was told by an experienced lady in a quilt shop to use the cornerstones even if they are the same color in order to make the sashing line up easier. I did this on my last quilt, after trying out different colors for the cornerstones. I like the way it turned out. The sashing is nice and straight. IMHO.

here is a tutorial on adding sashing the easy way.
http://www.synthcom.com/~val/Quilts/...ngSashing.html