When you shoot someone...

Would you ask for permission when you shoot a candid shot of someone? Sometimes, it's intriguing seeing someone doing something, and it captures the essence of life, but personally, I do wonder if the subject will take offence to it. Short of snapping with a zoom (which also runs the risk of having someone walk into your path!), what else can you do? Me a newbie, so would appreciate if the 'oldies' can share their experience.

Cheers!