When you’re not sure hello is called for maybe you should follow the 1st President’s lead and bobble instead. For there are individuals who you are less then friendly to you so a bob can satisfy the polite requirement without leading to moving the lips. This way your gesture is considerate but not so much that you have to worry about whether they think your “friends” now. If the bob leads to a peaceful conversation then that’s great and if it doesn’t you can just pretend you were adjusting your head instead. The head bob might be just the movement you need in your life.