Depending on the type of water heater you may still need to install a di-electric union. This is commonly used when you are joining two dissimilar metals. Most of the water heaters will have a galvanized steel nipple coming out of the top. When connecting a copper supply line to the galvanized pipe you are required to use a di-electric union to keep the materials from being in direct contact. Many water heaters such as Bradford White, have waterways built into the nipples coming off the top. These plastic waterways prevent the need for a di-electric union.
Overland Park Kansas does require thermal expansion tanks to be installed on all closed loop systems. A closed loop system is created when a type of backflow device is used to protect potable water supply. Inspectors in Overland Park are considering a PRV as a backflow device and thus creating a closed loop system. If you have a PRV, you will need a thermal expansion tank.