I set my sewing machine to sew about 12 stitches to the inch for most piecing - the setting is 2.5 on my Pfaff.
When sewing pieces together, I leave about two 'air' stitches on each the end of each sewn together unit
- this seems to minimize pulling apart.
I never backstitch when piecing. If I'm concerned about something coming apart, I tie a knot at the end of each seam.
When doing strip piecing, I shorten the stitch length to about 14 stitches to the inch (2.0 on machines that are set that way)