I think it all depends on your relationship with your boss, what sort of person they are, and what the company is like. I won't name my company, but it's a very old brand, very proud, and prides itself on integrity, commitment, and diversity. I had also asked in advance, anonymously (via the company Intranet anonymous Q/A page) whether a person transitioning in the workplace would be protected under the Company's anti-discrimination and harassment policy, and the answer was yes. So I was pretty sure the Company wouldn't turf me out due to transition. And I also have a really good boss, and a very good relationship with HER. Even normally, she meets weekly with each of her direct reports for a one-on-one session, just to find out how we're doing and what we're working on. So I scheduled an hour meeting instead of the normal half-hour, and said there was something I needed to discuss with her. When the meeting started, I started off by saying "This isn't going to be the usual one-on-one session..." and I also hastened to assure her that I wasn't quitting, and nobody had died or was sick. Then I just told her. I had some material from the web ready to hand to her about definitions, and I lent her a copy of True Selves. She was/is 100% supportive.
I think, though, a lot depends on who you're talking to. It isn't all about timing and what to say/how to say it.
