Well, at least in the U.S. you're legally required (although, some transpeople lie) to check off the female box in job applications if you haven't gotten your birth certificate changed. In some applications it's optional, but in other it's not.
So that would be another reason to inform an employer that you're trans. Sure, some people might not notice the female box, but others will. It'll cause problems pre-t and on T.
I honestly don't see anything wrong with informing an employer that you're trans. It's not like EVERYONE at work has to know or anything. If you're Pre-T and aren't passing very well, then it might be nice to have an employer who is trans friendly and who knows. Cause then they'll be respectful and use the right pronouns and let you use the right bathroom. If you apply to work as a girl, then you have A LOT of explaining to do if you go on T, but if they already know that you're trans, it won't be an issue.
If you're already on T then it's more of a dicey situation. If you want to be completely stealth, then sure you wouldn't want any one to know. It might be necessary in some situations if you haven't gotten your name changed, or if you check the female box and someone notices, or a myriad of other scenarios.
I'd rather work for an employer who is LGBT friendly, and just not have to worry about hiding every thing. That's a personal decision though, and i totally respect people who want to be stealth. I had a job where the person who hired me knows that i'm trans, and my boss knows that i'm trans, but no one else knew. That worked out pretty well for me. They were both cool with it, and very respectful.
If i made friends with one of my coworkers and i felt comfortable, then i might tell them, but it wasn't like everyone at work knew.
I'm pretty open about being trans, but again, that's a personal decision. It's not really related to work though, so i tend not to tell coworkers unless i'm friends with them. It's just not a big deal to me. Being stealth isn't something that i'm ever interested in doing.