Thanks a lot for giving additional ideas, how to ask.
My experience was most a good one. Only one person didn't want me to publish the picture after I have sent it to her (I have asked before taking the shot).
I expect that most feel honored to be asked because I give them personal attention. And I give them a reason why I want to take a shot especially from them in an open and kind way. Additionally I watch them for a while before I ask them. Some I didn't ask, because I'm quite sure, they wouldn't like to be photographed.
This are my tiny experience while taking shots from strangers.