"Dear Professor Smith" (or whatever his name is) should be your default. It's the form of address that will satisfy pretty much all of your professors, including the ones who want you to behave professionally. If your professor is starting off his emails to you with "Hi, Jack!!" you can start yours off with "Hi, Professor Smith!!" Try that with some professors, though, and you'll get a response along the lines of, "You need to write your email in a professional manner if you want me to answer your questions."