I generally gather with 2 rows of stitching because it is faster than 3 rows and looks almost as goos, but for the items I sell, I use three rows because it really looks better. Here’s an example. Row 1 has 1 row of stitching and you can see how it is a little lumpy. Row 2 has 2 rows of stitching and it looks pretty good. Row 3 has 3 rows of stitching and it also looks pretty good.
JOINING: To join your gathered edge fabric to the piece it will be sewn to, always line up the edges and put the non-gathered fabric on the bottom. This way, you can control how the gathered edge is going into the machine since it tries to move and pucker.