I often piece my backs and while I'm not trying to line up seams I do want the seams on the back to be straight with the quilt overall. I glue baste so the process might be a little different. I squiggle the glue on the batting and lay the backing over it. Once that is dry I'm ready to do the front. Before I flip the back/batting over to expose the batting I tape straws, pencils or whatever I have handy so there is a bump on the seam line that I can then align with a point of reference on the top. It is then easy to get both the front and back going straight. The quilt that I'm working on right now I put a large border on the back because I was short of backing fabric. It was really important that I get that on straight because it would be really obvious if one side of the border was wider etc. In this case I found a point of reference on my top that was the width of my desired border and used that to match up. I'm quilting the border from the back and it is turning out beautifully the way it matches to the front pattern of the quilt.
It's a little more work to do it this way but worth it in the end.