As to LAQ back tack: the quilt is put under some tension on the bars to keep it taunt-no wrinkles. This could pull out a seam that might be loose. So stitching around the top or back tack seams.
For double stitch; I like 50-60 wt thread so that when pressed the seams are crisp and aren't folding around a heavier wt thread. Putting in 2 rows of thread would compound the wt of the thread making the fold on the seam larger and potentially loosing more fabric in the fold. In the past, I often read to use a "skant 1/4" due to the fold over the thread. With 50-60 wt thread "skant" isn't necessary.
When my daughter started quilting at age 10, and I was a newbe at quilting, her seams were a little uneven and the quilts were under quilted. With use and laundry, the seams were tugged back and forth and did come apart. Once we got past that, there's been no problem with seams coming apart. We fixed the problem, not the symptoms.