in my experience, those who don't mind to be photographed, no words is required.
for those who mind, no matter how friendly I am or chat withthem beforehand or buy something first, they will always refuse.
waist level off-finder is the best I think
it is not only a polite or not polite question to subject
but that is the capture the most original scene (subject not pose in anyway)
I don't always place the camera at belly position (absolutely front).
If I need to get very close, I'll place the camera on hip side to form my body facing away from the subject.
In such case, the subject can hardly be aware of my shooting action.