It is not a rule of thumb to extract a wisdom tooth. Extracting or saving the wisdom teeth depends on the following : 1.alignment of your wisdom tooth with the adjacent molars 2. It's relation with the upper teeth. If these criteria are met, then we save the tooth. But if the tooth in the opposing jaw is missing or is out of standard jaw relation, then it is better to extract. Also if it's not in proper alignment with the adjacent molar, or if there is gum tissue or bone covering the tooth, it's better to extract as it will lead to infection due to various reasons. If your maintenance of a root canal treated tooth is proper, it will last life long