This question could be interpreted many different ways:
should my Database use a non-root user
Yes, you should create a non-root user (one for each unique app accessing the database to prevent your wordpress-db-user (do to some random wordpress exploit that comes out in the future) from having full access to your super-fancy-custom-web-apps-db and getting a full list of all of your users emails that you have been collecting.
should I create a user's table in my databse
Authentication or no ... this is kinda a no-brainer. If you want to be able to link
email@example.com to his preferred alias
super happy fun guy ... or one of your users decides that qmail sucks and they want to change there accounts email to firstname.lastname@example.org ... a users table with an internal UUID per user might be very useful.
do I need to store user-password-hashes for login
Yes and no ... putting full faith in some other websites authentication might save you from having to worry about having your local db hacked and having to report that a few thousand of your user's password hashes are out in the wild. But at the same time, if a vulnerability is found in this third party's authentication it could very well leave you in a worse situation since you have no control over the third party patching it ... and nothing to fall back on if this third party decides to charge for this auth service or just discontinue it all together.
This is more of a business decision than a security decision ... there are lots of pros and cons of going each way.