Our current dog is very careful about not biting the hands that feed her. We have had others which were dangerous. If they thought you had something they wanted to eat hidden in your hands they were fair game as far as they were concerned.
My first Shetland Sheepdog was a lovely girl. You could hold a treat out for her and she would take hold of it; but, if you did not let go of your end, she would let go and sit and wait for you to give it to her completely. My late father was always amazed that she wouldn't just grab it out of his hand.
Another thing my Dad liked to do was put butter or peanut butter (just a dab) onto a dog treat and give it to her. Once in a while he would pretend to do that and give it to her; but she would spit it out at him and give him a look like, what are you trying to pull here.