From a heating efficiency perspective, covering the entire pool is not necessary. The pool will still heat effectively even if it’s not entirely covered. However, leaving certain areas uncovered, such as edges along steps or other openings, can allow dirt and debris to enter the pool. Additionally, uncovered areas can provide a starting point for wind to lift the cover off the pool over time. For optimal performance and to keep the pool clean, it’s advisable to cover as much of the pool surface as possible with the pool cover.