I sewed the buttons on last after the quilting. On one quilt I took shallow stitches so the threads didn't go through the whole quilt. On the other quilt I poked that needle all the way through so you could see the tacks on the back. It worked out fine either way.
My sister says never put them on a quilt you're going to wrap up in. I decorated her quilt with buttons and she said they were very, very hard to sit on! :lol: I didn't do it on purpose, really! :lol: :lol: