My machine (Bernina and Pfaff) default at 2.5, I always set it down to 2. If paper piecing I usually go down to 1.5

I use 2.0 for piecing.

I use 2.0 and do not backstitch. I begin and end on a scrap of cloth so that I am cutting off a few stitches beyond the fabric.

The default on my machine is 2.2. If I choose the "piecing" stitch it is 1.8 mm. I generally use 2.0 mm. That is a little over 12 stitches per inch. The 1.8 is 14 stitches per inch and is a little dense for my taste. I never back stitch when piecing.

Ditto -- though I do backstitch if the seam will not be crossed by another seam.

If the tensions on your machine are balanced and the stitches are 10-12 an inch, seams coming undone shouldn't be a problem. I don't backstitch when piecing except Y seams.

1.8
1.4-1.6 for paper piecing

I sew with what I consider a pretty small stitch, too. I have a Featherweight that I do all my quilting on & I set the stitch at about 15 stitches per inch. That's pretty small, isn't it? I know it sure seems that way when I have to take those puppies back out... LOL!

1.9 I do not backstitch.

I generally use a bit longer than default for piecing, but if the pieces are small or the stitching will be cut (like in a bargello) I will use a smaller stitch. If your stitches are coming loose, it sounds like you are pulling against the seam. I never backstitch, but I too have had that issue, especially when the quilt top gets bigger and heavier. I have fixed that by stitching around the outside edge (1/8") catching all the seams. It won't show when the quilt is done and gives the piecing stability.