Awwww, what a little sweetheart she is! She reminds me A LOT of my Zoey (my favorite bird but I don't tell the others -LOL) and I tell you, getting bit or not getting bit depends 99.99% on the human's actions! I got Zoey when she was 6 or 7 years old, she had to be rehomed because her owner's wife gave him an ultimatum: her or the bird. Apparently, she used to be extremely aggressive toward her but, between you and me, the woman was rude and I did not like her either!
And Zoey has been super sweet with me from day one - she has never, ever nipped me, much less bite me! But she has bit other people (my husband and the daughter that cares for them on the very few occasions I go anywhere) but that's because they are one-person kind of birds and she felt they threatened her relationship to me - once she figured out they were just members of the flock and no real competition, she stopped.
If I may make a suggestion? Use a phrase to indicate the imminent scratching of the head. I use "pica pica" but you can use any word or phrase you feel comfortable with and you will see that, in time, she will recognize the word/phrase as a request for permission to scratch her head and either grant it or not (depending on her mood)... she might even use the phrase herself to ask for it (all my birds understand what Pica Pica means but I also have two that use the words to ask me for a head scratch (one of them, Sweetpea, the male Senegal, actually asks Zoey to allow him to preen her with the words but she never accepts his offer, poor Sweetpea!)