Of course it's acceptable! It's your quilt & your decision. The only thing I might suggest is adding an extra 1/4" of the brown to the top so when you apply your binding (assuming you're using traditional 1/4" binding) so that way you still will see the full width of the sashing. Of course, it's completely your decision & if you're fine with skinnier sashing on the top, that's a design choice and I fully support it! At least with matching brown binding, it will blend together.