Even if your doctor says she doesn't need them now, I think a small amount is very good when she is older (6 mo.+). Especially vit. D & E and Omega 3 oils. It can't hurt, as long as her levels don't get too high. And as long as you don't give her too much, the healthier her body is, the better to fight off infection. If she has extra reserves of vitamins/minerals to draw on when or if she gets sick, or at times when she might not be eating a lot, then she will be ahead of the game.
That is just my opinion. I really believe in going the extra mile when it comes to nutrition. It can be hard to get the necessary dosages of nutrients that a person with CF needs in order to thrive, especially when eating a normal american diet and not taking supplemental vitamins (let alone when you have to deal with a child's pickiness!). They not only have malabsorption to deal with, but constantly fighting bacteria puts a heavy load on their body, and they use up nutrients much more quickly than a person w/out CF.
-Willow, mom to Cheyenne, 3, w/CF