Okay, this one is for college students and recent grads.
Someone along the way, someone gave you the idea that it's unprofessional to use anything but your full, given name, even if everyone in your life calls you something else. While this is sort of adorable, you should drop this idea.
It's disconcerting to go through the whole interview process with Katherine and hire her, only to discover on her second day of work that she's Katie. Or weirder, to discover that the William who I spent several weeks talking with, and who I now know as William, actually goes by his middle name, Peter.
There's nothing unprofessional about middle names, or about Katie, Jim, Liz, or whatever your nickname is. This isn't like school where you have to register with your full and complete birth name. We're all adults here; just tell me what to call you and don't confuse me.