Legally, from what I remember reading on the state website, a person can legally register a car in any county that they own a buisness, or house. For those larger buisness's, if they are company cars, they can be registered wherever they have an office, but most likely, where ever the home office is, or, wherever the vehicle was originally purchased, but may have been moved to a different branch.
However, there are many people that will register a vehicle at a family members address, just for the tax savings. This, I believe, is illegal.
Here are the faq's from the state website.
Kansas Department of Revenue - Frequently Asked Questions - Motor Vehicles - Titles and Registrations