OK, maybe I'm asking the obvious, but I've never seen this addressed in a book, and our teacher never told us to do it in our class.
The first few quilts we stretched for quilting didn't stretch well. There was extra fabric in spots; not horrible, but enough for us to notice, even though we stretched them on the DR table and used binder clips to keep them taut. On the last three that we have done, I looked at the batting as we took it out of the package and it had all kinds of wrinkles and waves. So, I took them to the ironing board and pressed them with steam. Those quilts stretched beautifully and had no hint of bunching after they were quilted. This is definitely going to become an item on our checklist going forward.