If you are entertaining the long arm quilter route, do not baste the quilt together. It will just have to be taken apart and probably the LA'er will charge you for doing so. The top, and back have to be put on separate rollers and the batting floated between the top and back layers. You might sew a piece of muslin to the sides and that would give them something to attatch the top and bottom leaders. The sides need that extra to clear the clamps. Depending on how you want it quilted and the batting thickness, you will need to have at least a 4" strip on each side. If you are doing custom and the LA'er will be using a table and ruler, you will need 6" on each side.
I know most quilters do not understand the why's of having that extra yardage on the back, I would suggest go to a long arm shop and ask to watch them load a quilt and it will become quickly apparent as to why that extra yardage is needed. Hope this sheds some light on the subject.