I told people in advance. Had I everything to do over again, I would have used delay tactics, or just disappear, but I don't think it was possible for me to know this at that time, back in the day. Or, I dunno, maybe I just did what I had to do.
The thing about coming out is that it's a ritual, whereby you ask people to treat you differently. And early in transition, this is probably the only way to start getting gendered correctly. But how good people actually are at this, well, it varies. Some are spiritual masters and get it right away. Others dig in their heels and resist, regardless of what you do. A few can make the jump, but only after you've changed your body enough to naturally elicit female gendering in the first place. And some very well-meaning people will be initially supportive, but struggle to get it right and slowly develop an antipathy to the whole endeavor.
I think it really depends on how you think your life is going to go. If you're young, you don't have kids, and you plan on going all the way to where you wouldn't get clocked unless you outed yourself, well... it might be best to just tell a few trusted family members and hole up in the trans community until you're ready to break out. Because in my experience, very few people who knew me from before were even capable of gendering me properly; everyone else I had to leave behind anyways. And it would have been so much easier, in hindsight, to simply ghost out.