Well, it’ll be for you to judge and is best to see your dentist if concerned. Generally, crowns are needed to support the tooth in function and longevity. So, post-root canals, especially in molars, if less than 2 mm of remaining tooth structure encase the tooth, fractured tooth with a missing cusp, teeth which are malformed and unable to remedy using resin, etc., these are a few of the signs one would look for to judge the need for a crown.
Hope this helped.
If the tooth needs a root canal a crown restauration is indicated to cover and protect the tooth.