Funny story. I was checking into the hospital earlier this week (ok, that's not the funny part, that part sucks), and going over my information with the clerk. She kept referring to "me" as "him" and "the patient". She had no idea I was actually the person being admitted as she never asked me what MY name was. So, when we got down to the next of kin contact section, it was a woman's name listed of course, my wife. She asked if that was me and when I said no she got all embarrassed and asked if I was the girlfriend or something! LOL. She really couldn't tell that I was male at all and took some convincing for sure. I tried to show her my ID at the beginning of our conversation but she dismissed it saying she didn't need it.
So anyways, in cases of "legal" issues or insurance or things requiring identity documentation you should go with your real name of course. But if you're presenting as a different gender from your documentation then feel free to give them a "preferred" name to use. Not always necessary, but a good idea if you'll be around them a while or they'll be calling your name out over a PA system or something, like at the doctor's office. They have my real name on file and on all the important documents, but I have a preferred name they use for interacting with me. When they fail to use my preferred name, I'll just ignore them and wait for them to figure it out and try again, lol.
I am going to start using my new name for everything I can. There's no reason why you can't for most things. I mean like people who go by "Bubba". That's hopefully not their real name, but people still use it!!!