Adding SSO doesn't mean everything must use the sso. I would implement it at the start. It can also make things easier for the companies that don't tax the sso. So the login to services is faster and offboarding is also easier for sso enabled apps. edit: misunderstood the question.
Found discussion here: https://old.reddit.com/r/sysadmin/comments/g18m6e/when_to_co...