I didn't tell absolutely everyone, down to mere acquaintances, but I did tell all those most likely to notice and be affected by my illness - i.e. family, close friends, and work.
I could have chosen only to tell boss/HR, but as I thought immediate colleagues were likely to have to shoulder extra work, as a result of what might have looked like laziness, they were owed a proper explanation.
Overall, I have found it of benefit to be frank with people. And in some cases, there was no choice - for example to claim on my critical illness policy, which was part of my work benefits package, there didn't seem to be any route except to go through HR! You can hardly make a critical illness claim without even the least bright HR bod working out that you must have a critical illness.
The only thing I would say is I'm not sure what a "working diagnosis" is - it sounds short of a confirmed diagnosis. There is a remote chance you'd end up with egg on your face, if you went round telling everyone "it's MS", but the "working diagnosis" was later jettisoned.
Now I don't think consultants go round introducing the idea that somebody has MS, unless they're pretty sure. You wouldn't alarm a patient unnecessarily, would you? So when my consultant told me MS was "the most likely" - months before my official diagnosis - I knew he must already be fairly confident, otherwise he wouldn't even have mentioned it! So I spent the next six months knowing, but being technically unable to prove it, until further scans led to my confirmed diagnosis.
So I don't think it's very likely your diagnosis is going to change. However, unless and until it's confirmed, there's always that small possibility. The way my consultant put it: "Common things are common; rare things are rare".
By which he meant that MS, being the commonest disabling neurological condition amoung young people was far more likely to be the explanation than certain other rare and weird conditions that can produce similar signs and symptoms.
However, you need to be aware there ARE rare things that can look similar, so until you're told 100% it's MS, there's always the slight possibility they might revise the diagnosis to one of those rare things.
Could be embarrassing if you've already told everyone it's MS! So perhaps the best course is complete honesty. Rather than saying, as a certainty, "I have MS", perhaps you could say: "It's virtually certain it's MS now - they've ruled out most other things". That way, if your consultant did come back later, and say, in the light of all the evidence, he's changed his mind, you wouldn't be left looking silly - or worse still, a liar.