I'm in the place you're at with mitered corners. I'm using a stripe for the sashing so trying to get both sides to match up is my problem. So decided to sew just the corners up to the 1/4" seam and about 10 inches the other direction, then I'll measure and seam the 2 ends together in the middle making sure there's no fullness. Luckily I have a border to add to it afterwards so that will take out any fullness I might have made on the sashing. The border will also be mitered only because the sashing is mitered. Think it looks better when you make both the sashing and border mitered instead of only one or the other..............just my opinion though.