I sew, but I know most just glue. I guess it all depends on the glue that you are using and if the layers come apart. The one time I glued the customer said her little girl pulled it apart right off the bat and I have never glued since then. Lol. I save all the long little stem things that go through the centers of the flowers (I save only the ones with the flat nail head type thing on the end). I put all my layers together on that thing to hold them together and then I sew 5 simple stitches around the center of the flower.
For rolled rosettes and other such flowers I just use glue because I sew the flower part as I make it.