Do you care about getting along with your co-workers? I mean they need to give you respect but you also ought to be respectful of the position they are being put in. I'm not talking about dealing with your decision to transition, that shouldn't be a problem and if it is you shouldn't care.
However if I was at work and someone transitioned and I had to listen to snide comments from other colleagues it would really be very upsetting to me and piss me off. So it could interfere with my ability to focus on the job.
So what about the people who are probably going to have to contend with the gossip and behind the scenes ugliness? I agree that you should let HR know so they can prepare and also let the staff you work closely with know.
Good luck.