Like PP I have done both. But I don't base my decision on the squareness/flatness, I base it on thread changes. If I am going to ditch all in one thread and background stitch in another I will ditch the whole quilt and then roll it back and do the background work. If both are done in the same thread I do it as I go. But if I am doing it as I go, I will ditch the entire width that is exposed in that section of frame then do the detail.