I have been fortunate with this piece ( IPM ) in that there has not been too much distortion. I didn't use a heavy canvas this time because I wanted to see how difficult it would be to comply with Quilt Show rules that insist the stitching be visible on the back. For commercial work I really don't care that I cover the work but for a juried show it can make the difference in a tie!
The nature of this piece had me stitching lightly at first over all area, tacking down pieces and added small elements here and there. It wasn't until I added the heavy felted trees that I started noticing some issues with the edges. The more I stitched them the more pronounce it became.
Some of it disappeared as I added the thin trees on both sides. The right side was almost gone but it was still pronounced on the left.
The reason for it is simple. The repeated lines of stitch vertically along the edge were drawing it in. Every time you change weight of thread or restitch an area this increases the density of the stitching. It affects how the set tension of the machine lays those stitches.
Here is a "cheap and dirty" solution....and another reason to over size your work. An inch or two all around is all that's necessary.
I ran a series of stitches up and down over a concentrated area in the region where the wowing was showing. This drew in the offending spot and now the edge lays flat. Before I trim the piece to its final shape I'll run a few more lines the whole length. These will be hidden under which every finish I decide on.