I'm a little OCD and would want my backing to line up too, if it was pieced. I have done it before in a couple quilts and like PaperPrincess used pins (but like her idea of thumb tacks better), HOWEVER I spray baste so it is a little easier. I spray baste the batting to the backing and then mark the matching points and carefully lay my top down to square it up with the backing. I then carefully roll half the quilt top on a squared off board my husband cut from MDF (3" x the closest width of the quilt. I have 4 ft, 5 ft, 6 ft, 7 ft, and 8 ft) and spray baste that half back in position. Then repeat for the other half. I then pin the entire perimeter and take it back to the sewing machine and baste 1/4" inside the edge to stabilize and prevent the top from peeling away as I quilt. I did the following in a meander on my DSM with a 6 1/2" throat, so it isn't a large quilt, but it lines up and it was not difficult to quilt with double seam allowances. Good luck! One of my quilts can be viewed here:
http://www.quiltingboard.com/picture...t-t236711.html