Best practice to use identity_url

Reading the google oid best practices document available on
the web, it argues that you shouldn't use email address as your
users database primary key, because email addresses can expire
and get re-used, among other reasons.

Looking around, it looks like flask oid's resp object 'identity_url'
is actually the unique identifier to use.  It is slightly unclear to
me if this will move around based on the requesting url.