I piece a lot of my backings. Usually with leftovers from the top. However, I quilt on a LA so have no issues with bulk when hitting an area with all those seam allowances. This is something you may wish to also take into consideration. As far as perfectly lining up the back to the front, I have never done it successfully. I am always a bit off. I would never attempt to do a double sided quilt where blocks need to line up exactly, however it has been done with great success. I prefer to not give myself the headache so my pieced backings will be completely different design than the front as in this example.
Here is the front:
[ATTACH=CONFIG]529238[/ATTACH]
And this is the back
[ATTACH=CONFIG]529239[/ATTACH]