So here's exactly where I think the line on politeness must be drawn. It's that point, when a person, supposedly asking for a favour, automatically places on you the burden of necessarily acquiescing to it. Otherwise, you risk being certified a douche. No, seriously.

Circumstances may vary. But I've been inevitably finding myself in a spot over the past year or so, when more often than not, someone whose eyes have barely met mine at a cafe/bar/airport/event — mostly in Mumbai, where movies are made — demands my phone number, or email id, as a fundamental right.