I've heard you legally have to write down what your birth certificate says, I've tried looking around a tad, but I haven't found anything. Something that there's a law stating you can write down your preferred name, or gender. I was just wondering since I know you can do that with a drivers license in New Jersey.
In most states you have to be honest and truthful......which means you have to give your legal name and what gender you are, not what you want to be. You can get your name changed legally and your sex reassigned. But you have to report them as they are.
You definitely have to use your legal name for employment purposes. But you can also change your legal name without tremendous difficulty. I'm not so sure how easy it is to change your gender identity on things like drivers licenses and such.
I believe the legal gender changes require some sort of medical and/or court-declared confirmation, don't quote me on that. It varies from state to state, so look it up for your specific state. A MtF friend recently got the "Female" driver's license after 2 years of hormone therapy (she's at the point where all that's left is the genital surgery.)