Stockinette curls. It's just what it does. It hasn't anything to do with whether you increase or not at the edges -- increasing just shapes the piece so it fits your body or creates a certain shape. If you were supposed to increase and you didn't, you may find that the fit is different from what it was meant to be.
As Marilyn says, if you have to sew the pieces together, you don't have to worry about the edges curling; the seam will do that.
But you may need to check that the garment isn't too small in places. There are different methods to cast on to your needle at the beginning, end and middle of an existing row; I like knitting on, aka a cable cast-on. You can also increase somewhere within the row.
The video tab above is worth exploring, and you might also try rummaging through the Techknitting blog.