I waited until after graduation, which was relatively soon after my viva and minor corrections.
But the key point is when your corrections are approved, and when the resulting paperwork has gone through uni authorities afterwards (Senate or whatever). That happens before graduation.
I notified my bank of my PhD and new title because it's useful for me for ID purposes. But otherwise I don't use the title outside academia/research. Though I'm working as an independent researcher (albeit one with an honorary university affiliation), so not in a university environment.