I am a retired RCN Reserve flag officer. The only time I use my rank with the postscript (ret), is when responding to official military correspondence, such as invitation to official events. I still start e-mails and letters to certain flag officers who I greatly respect with the the honorific "sir" and will continue to do so as a mark of my respect. A few officers who have worked for me do the same when addressing me. I tell them it is not necessary but most continue the practice. I would like to think that is because I have earned their respect and trust.
I never use my rank or even share the fact that I have served in the military with others unless directly asked the question. There is nothing, in my opinion, more annoying than retired officers who think they still matter.