My theory is that the backing should be no more "taut" than the top - because if it is pulled too tight - then when the tension is taken off, it will pucker up.
When I lay backing on my table, I "weight" the sides that hang over with metal yardsticks - I make a temporary "tube/pocket" to hold the yardsticks and that creates enough "tension" to minimize wrinkles on the backing. I do tape the edges that are on the table - but I "smooth" - I do not "pull" the fabric.