I guess it depends on how you want to load your quilt. (I don't use zippers, so take this information with a grain of salt.) You would want zippers for the top and bottom of the backing for sure. If you float your top, that's it. I don't think anyone attaches the top of the quilt top to a leader - so you don't need one there. If you attach the bottom of your quilt top to a leader, you would have a zipper there. If you want the zippers because you will be taking one unfinished quilt off the frame in order to work on a different quilt, then you will need additional "half" zippers for each quilt that has been taken off. There's a lot of information about the zipper system here -
http://stores.longarmconnection.com/quick-zip-system/