Allyson discusses the importance of proper planning and placement of PEX runs for radiant flooring systems. It is essential to ensure that the PEX runs are within the recommended threshold of 280 to 300 linear feet to maintain even heating throughout the floor. Additionally, it is important to consider placing the PEX runs in areas where you will be walking to avoid cold spots and ensure a comfortable temperature. Care should also be taken to avoid running the PEX too close to drains or other fixtures that could be affected by heat. Custom routing may be necessary in certain areas to optimize the heating system's effectiveness. Remember, the goal is to have a consistently warm floor where you will be walking or standing.