As a long haul trucker for many years I have enjoyed the wonderful opportunity in meeting and photographing many people along my travels. I have been asked several times if I ever ask permission before taking street portraits and the answer is sometimes.
It really depends on the feeling I get with each individual. Obviously asking some stranger if they mind if you take their picture will affect the outcome of their expression but I usually talk with the person for a while in a friendly way before asking them, camera in hand. There are times of course, I would have missed a good candid shot had I stopped to ask if they objected.
In my opinion, it's how you carry yourself and your camera that allows your subjects to feel comfortable, or not. I've heard you convey your self image through your lens to your subject and it is this reflected image you receive back on a subconscious level from your subject.
I carry model releases with me and have only used them on occasion. I hand out printed 4x6 calling cards folded in half with varied images including my contact info and where they can view their images in the following week. I show them the in camera image and always tell them how wonderful their picture came out and if they are interested I will happily email them a copy in full resolution. I follow up with all requests.
My favorite work and what I enjoy the most is photographing the landscape of America, particularly the American West, but I will always enjoy the challenge of shooting strangers and learning something new from all those I photograph. Shoot a stranger, make a friend!
An interesting article by Qiana Mestrich titled "Photography Empathy: How you feel is what you get" about how photographers are like mirrors can be read at Black Star.
"As I progressed further with my project, it became obvious that it was really unimportant where I chose to photograph. The particular place simply provided an excuse to produce work... you can only see what you are ready to see - what mirrors your mind at that particular time." ~George Tice