Well, it all depends on the time of day, sex of the stranger, age, appearance, district (where we met), my mood, and what was written in my horoscope this morning...
Honestly, I have a strange habit. When a stranger approaching with some question, I immediately say no and go away, even if he has not started talking. Later, my brain can have time to process the request, which flew in my back.
My high-speed biological neural processors assess the level of threat and if everything is OK, I'm going back to help, straining cheekbones to look more masculine (cuz I don't have a brawny arm like you).
When it's all over and we disperse, I start to think who was more terrified, and whose teeth were stronger during the straining cheekbones. Therefore, I prefer to look back for the next 10 minutes, what if this stranger decide to take rematch.
Conclusion: Help people with a smile on your paralyzed face. So you can save more teeth and will not bite your tongue, when the stranger decide to do the upper-cut to your innocent face.