When I was hired at Panera I kept it 'under wraps' until one day at clinic they wanted to admit me to the hospital. Lets say I felt terrible having to call out for two weeks without any prior notice/warning. They were very understanding and took me off the schedule until 'further notice' allowing me as much time as needed... but I still felt really guilty doing that to them. When I was interviewing for my second job at Bebe, I told my boss right up front that I have Cystic Fibrosis and every once in a while will need days off for clinic appts. and every once in a while two weeks for a tune-up. She was very very understanding and was very accommodating, for example, knowing I had CF, on the days I wasn't feeling my best she would let me take over fitting room duty and take a chair from the back and allow me to sit while running the fitting rooms, and having another employee do 'go-backs' around the store, so I wouldn't have to walk as much.
After those jobs, I started to Nanny and have always been upfront with every family I nanny for, letting them know that I cough and will need days off or 'slow days' etc etc. I've got nothing but positive feedback being honest.
AND just so you know, if they don't hire you BECAUSE you have CF then that is illegal and discriminatory against people with disabilities. It's like not hiring someone just because they are in a wheelchair, or whatever.
If I had to interview again for another job job, not a nannying job, then I would probably interview and keep it under wraps, but then if they extend an offer of the job to me, then I would say,"I would love to take the job, but I would like to inform you that I do have Cystic Fibrosis blah blah blah" That way you've already sealed the deal for the job, but you are also being honest and they will most likely be more accommodating towards you. =) GOOD LUCK!