IPhone prices typically drop once a newer model has been announced. It is unusual for the price of any available iPhone model to change prior to this announcement.
Once the newest model has been announced, the previous generation often becomes discounted; however, there may be less options available for the older series. For example, when Apple announced the iPhone 6, the set of available discounted iPhone 5c models were restricted in terms of internal storage space. While the 5c previously came with 16 and 32 gigabyte options, the company reduced the available options to just the eight gigabyte model. This only affects models purchased directly from the company, as used 5c series models may be sold by individuals with the larger storage capacity. Thus, discounts occurring after the announcement of the latest model sometimes come with relevant restrictions.
The specifics on the number of models sold directly from the Apple company at any one time varies. With the announcement of iPhone 6, Apple only directly sold four models to customers: the iPhone 6, 6 Plus, 5c and 5s. The announcement of the iPhone 6 also led to the discontinuation of the iPhone 4S. Thus, waiting for the price drop at the time of a new model announcement runs the risk of being unable to purchase the particular model of interest.