Well I'm 30 years downstream from SRS, and despite being partially anatomically male at the time, I grew up as mostly female thanks to parents so far ahead of their time they were almost in the next galaxy!!!
So obligation to tell? NOPE!
Would I? YEP!
When? Quite early on actually and well before we move beyond casual friendship.
Why? Because I want to test their reaction to find out whether they are worthy of my affections or even interest. If they react badly I'm going to eject them from my life so forcefully they are going to wonder what the heck just hit them. If they react with a reasonable or questioning response THEN and
only then do we have anything worth wasting my time on. So I don't worry about them rejecting me, but they sure as heck better worry that I won't reject them.
Basically I'm almost arrogant enough to think that rare people like us are special and superior and we don't want to be wasting our time on a boring "norm" who isn't worthy of it! 
(oh and please note the wink!)
So does this mean I am fully out and open? NOPE!
Explain...? Well at work or in casual social situations I don't see the need to tell anyone about something which happened so long ago that most of them weren't even born. The only exception I make to this is if I think that telling someone will serve a useful purpose, such as for example enabling me to educate my fellow medical students about how to react to something like that - and of course to show that that with some of us, and contrary to popular belief, there simply are no tell tale signs...
But your signature says you identify as CIS - how does that work? Well I didn't experience gender dysphoria, I was intersex, I didn't have much difficulty getting my treatment and after this long I certainly don't have any issues - I also tend to assume most cis privileges, so its not a perfect fit, but then nor are any of the available labels and cis happens to be the closest I can find. Also it is a tenet of this site that people have the right to self identify as they wish.