Sounds to me like most folks who have amazons don't think the shoulder is a good place. I agree. My long time pet hen nape prefers being on my arm/hand. She'll retreat to my shoulder if she thinks i'm going to put her away.She knows i have much less control over her and it's harder to reach her. I do put her on my shoulder at times when i need my hands free (and don't have my neck perch on),but it's not her favorite place to be. She bit me not too long ago while on my shoulder. I was at the teller (at the bank) and put up there because i needed my hands free. Someone behind me thought they'd try and pet her, bad move for me. I've had her for 43 yrs and she's the sweetest bird, but when on your shoulder ,you never know what's going on with them.
I've learned working with rescue zons, what the ones who have a distrust of hands are quick to climb to your shoulder. This is to get away from the hands and to have more control over their situation. It will take loads of time and patience for you to gain her trust, especial of your hands. Teach her that hands are her friend, easier said then done, but so worth the trouble.
The best way to prevent her going to your shoulder??? I'd NEVER allow it. If she goes there , remove her at once. Be consistent, be firm and don't cave in. When she's showing interest in being on your shoulder, get her on your hand, reward her with a favorite activity (pets/scratches, etc). I usually spend a few moments petting/cuddling (lots of attention) when i first pick her up on my hand. It's very similar to keeping your bird away from places you don't want her. Don't allow it and reward her when she does right. Give her a better place to be ( in her mind). Good luck and remember , your both still learning about each other and zons respect a good leader. Be a good leader and show her the rules to your flock.
PS i would not have answered this thread if someone hadn't shown a picture of a Vet with birds on their shoulder. I feel the forum members did a great job in responding and didn't need me to reinforce their comments. I feel certain that if you asked that Vet, she would tell someone in your situation that it's not a good idea. Don't be mislead.