Kuriboh, are you speaking of the mouth-plate? I usually sew mine in. It really depends on the method of puppet-building you're using, I think. For example, I believe that the foam book has you make your mouth-pate & then build the puppet around it. Its been awhile since Ive done it that way.
The glorified sock puppet from projectpuppet.com has you sew the fabric of the mouth in first, then attach the mouthplate later. that's always been the way I'd done it, so it was good to see it in the pattern, too.