The decision largely rests on your personal preference. If your aim is to gain insights into the history and importance of Karnak, I would suggest hiring a guide. They can provide valuable knowledge about the various sections of the temple complex and share information about the prominent figures involved in its construction.
On the other hand, if your interest lies in exploring Karnak independently and absorbing its ambiance, a guide may not be necessary. The temple complex features numerous signs and information boards that offer details about the different structures, allowing you to learn at your own pace.