I do shorten the stitch when starting and stopping, to .6 and do like longarm quilting, with about 6 stitches being secure enough to hold without back stitching. Sometimes I do a little back stitch, but it doesn't show as much with a .6 length.

I do quite a bit of stitching in the ditch, and I too lock stitch before starting/stopping. I stick to the default stitch length on my machine (2.5) because I find I forget to lengthen the length when I start a new session. And yes, there are wobbles along the way. Taken as a whole, those wobbles disappear when finished and washed. Definitely use a walking foot, and pin baste very well.

When I stitch-in-the-ditch on my domestic machine, I use a walking foot and the standard stitch length (because, I too forget to reset it otherwise). I will stitch near the ditch when the fabrics are high contrast and I want to always stitch in the side that coordinates with my thread, even if the ditch is on the other side. Unless they extend beyond the edge of the quilt, I bury my threads (because I am good at it and what’s a little extra time when you already quilt as slowly as I do?).

I stitch in the side of the ditch. The width of my machine foot. I keep all stitch lines on the same side of the seam. It takes too much effort to keep all the stitches right in the ditch, a few wobbles and it looks sloppy. I use a three setting for length.