That is one of those issues that varies considerably from one doctor to another. Some are very agressive and treat it as soon as it is cultured. Others don't treat it unless it is causing symptoms.
I have two daughters with CF and we've been to several CF Centers over the years. The majority of our time has been spent at the Minnesota CF Center in Minneapolis and the San Antonio CF Center. The Minnesota doctors were very agressive and treated staph once it was cultured regardless of symptoms. The San Antonio clinic does not treat it unless it is causing problems.
The reason usually given for not treating it is that giving antibiotics when there are no symptoms causes the bacteria to build up resistance to the antibiotics and then when the patient really is sick and having symptoms, the antibiotics will not work.
If your daughter has not been on antibiotics a lot, it would not hurt to try a course of them and see if it takes care of the staph. You always have the right to call the doctor and express your concerns and ask them to give her an antibiotic even though that isn't their normal course of action. Of course, they may say no, but if you are concerned it is most certainly worth a try. I tend to err on the side of caution as well and would probably want to treat it right away. If it persists and she continues to culture it anyway, then I'd back off until is actually causes problems.
Good luck!!!