To be able to assign licenses to all current and future subscriptions, while minimizing the administrative effort, one should apply the role to the Root Management Group.
And because we should use the principle of least privilege we should chose the User Access Administrator role instead of the Owner one.
Reference:
https://docs.microsoft.com/en-us/azure/role-based-access-control/rbac-and-directory-admin-roles
https://docs.microsoft.com/en-us/azure/governance/management-groups/overview