Most people seem to do all the SitD first, but I find that sometimes if I do this, I end up with puckers because the fabric gets pressed to the outside as I do my FMQ in the "box" I created with my SitD. Therefore, I start in the centre and just do what is needed for what I'm working on. That way I feel the top can expand outwards as I FMQ my design without running up against barriers made by ditches.
I spray baste and never have a problem with shifting.
Watson