The initial cast application is included in the fracture care, but subsequent castings may be reported if medically necessary. For example, if the patient loses muscle mass and the cast gets to loose then it would be appropriate to report another casting application. However, if the cast gets dirty and smells bad and the patient wants it changed, then this is not medically necessary. The supplies are always billable for the supplier.
This is all explained in the CPT® book in the surgery/MS section.
Yes, your office can choose to either report the fracture care codes or itemize the visits. The AAOS has an article on this here: http://www.aaos.org/news/aaosnow/jul08/managing2.asp