The US has many different organizational blood suppliers. While some organizations are national like the American Rec Cross (ARC) there are many regional and even local organizations. In my experience, each center has their own screening policy, which is determined by their hospitals requirements. A region with a high sickle cell population may send all (or most) new African American donors for molecular testing, while other regions may only screen units when specificities are needed.
So, when a blood center has an aggressive screening policy, or when they are looking for specific phenotype may affect the frequencies hospitals encounter. This explains Cliff’s and my experience described above. Also, the local donor population, and/or if (and when) the blood center imports units from a different region may impact the antigen frequencies hospitals encounter .
As described by other posters above, I use the antigen frequencies to primarily determine the order in which to screen antigens and to manage expectations. For example, when I need to screen for R2R2 K- units there is an approximate expectation of 2%. I would therefore screen batches of 100 units; on one occasion I found zero units, on another 9 units with the norm being between 1 and 3 units. I am jealous screening for R2R2 K- units (or any Rh combo) would not be needed in the UK!!