Once it is sandwiched, I don't think it will make much difference. I did however want to have my sashing to have the same feel as my foundation pieced blocks on a top I did. I just cut 1 inch sashing and sewing 1/4 inch on both side seams brings the visible sashing down to 1/2 inch and the seam allowances add the extra thickness underneath.
P.S. It also gave the thicker edges of the foundation blocks somewhere to go as I pressed them all into the sashing.
Last edited by Tartan; 07-17-2014 at 09:17 AM.