From a perspective of overall lowest cost, I would recommend a proper service at around 50,000 miles. A proper service consist of test drive, inspection, fluid and filter replacement, with KIA recommended fluid.
Not to be confused with a transmission flush, that is widely marketed. KIA warns against transmission flushing (Bulletin Number KT2008082501) and flushing should not be done. The cost of a proper service is a fraction of transmission repair/replacment. Here is more on why to avoid transmission flushing