It depends on what you are trying to capture at the end of the day. Often if you are photographing a split second of expression that can't be duplicated in a posed photo you don't have a choice.

If you're taking a photo of a bum as you so eloquently put it- speak to them, find out why they are there- connect with your subject and then capture the emotion as you notice it.

If you are taking pictures of people on the street you are bound to get people fired up. Search YouTube for street shots Bruce Gilden. You will see what I mean.