Pullling the backing really really taut will cause more problems. Backing should only be smoothed out, not pulled taut. If pulled too taut while sandwiching, once the tape is released the backing fabric will spring back and cause puckering.
My advice is to heavily starch the backing fabric before sandwiching. A 1:1 solution of Sta-Flo liquid laundry starch and water is a good proportion. This is much heavier than spray starching. It is also a good idea to apply several layers of spray starch to the top. What the starch does is *stabilize* the fabric so it is much less likely to stretch, distort, or pucker while you quilt.
505 is good because it creates continuous contact between the 3 sandwich layers.
I personally do not like sandwiching on a wall, as I find it more difficult than sandwiching on a table.