Hello Lowboy, I do not have an issue with teeth but, when I play through a duck bill type of mouth piece I get more strain on my upper lip. I find that I can play a lot longer with a trumpet mouth piece. That is why I changed from the stiff flexible pipe on the Hammond melodeons to the trumpet. I tend to bite down on the duck bill type mouth piece, that may be why my upper lip gets stiff.
Melodica-Me