The straight of grain binding will not go around the corners well (the binding will have puckers) I have used straight of grain and bias on a quilt before. I pinned my straight of grain binding up to the curve, sewed on a small length of bias and proceeded to do the remaining quilt in the same manner. If you have a dark or busy print binding, you can't really tell where the seams are. Would it be worth it on a placemat? Not to me. I would just cut bias binding. Don't forget you have bias binding already, if you have some matching ties to recycle.