I'm not transgender, but if I was I would find that very offensive.
Call a transgender person the name they identify with. It's up to them to decide...not you since you can't know how they feel or how they think.
Would you like someone to call you questionable names or things that make you seem less than you are? How about if you're a guy, someone calls you she? Or if you're a girl, someone calls you a guy, boy, etc. Or worse yet, how about IT? Would you find that acceptable?
If not, remember that you can be discriminated against just as easily as others discriminate against LGBT. No matter who you are or what you think of yourself, there is always someone out there who can tear you down for no reason other than it gives them pleasure to see you reduced to a name said in cruelty and ignorance, even if they don't know you.
Be respectful. Transgender people have a hard enough time in love figuring things out and transitioning and all that. It's much more important for you to be concerned with being a decent human being.