I learned PPing from amma's tute here on the board. When joining the units together, I stick an appliqué pin - the tiny little ones, upright and right through all the seam junctions, then when the foot is approaching it, I take it out. I have heard about the glue but not tried it yet. I think that amma said she also sometimes just takes a tiny tacking stitch by hand through all the junctions, if it's an especially tricky join.