For me they want to avoid a c section at all costs so I can do my chest pt/vest right after I deliver-like Lauren said- my docs and my obgyn both told me they'll want to avoid a c section till the last possible minute b/c of coughing/chest pt reasons.
I am going for a non medicated/natural vaginal birth with a doula-(birthing assistant), but whatever is best for the baby is what will happen. If something comes up, I will be flexible to change my birth plan accordingly.
As to breathing during labor, I think that the lamaze or bradley's technique birthing classes can help greatly esp for cfers to learn how to do controlled calm breathing. And I know my doc mentioned that if I am having trouble, they will give me oxygen, albuterol or pulmicort to help me to breathe during labor/pushing.
Like Lauren said, this is why it is so good to have a cf doc that has some experience with cf moms giving birth. They just know what to do if something comes up. I hope you find someone to help you, and wish you the best.