I would do the same as you ... SITD all the sashings first.
However, I would do all ... including the outside edge of sashing.
It's the same fabric, and same look, so the same treatment at the same time.
That should keep your sashings lined up well.
A couple of other thoughts about potential wonky-causes .....
* Do you check as you make the sandwich that the sashings are straight and square?
* Because I see you have pinned your sandwich ... does shifting happening, as you handle the quilt, while you do the SITD step?