I am a photographer who routinely goes out onto the streets in Salt Lake City Utah and will ask people if I can photograph them, do I need a release?

As a portrait photographer, I love to go out in public with a hand held flash and an umbrella, and ask strangers if I may take their portraits. In General, I am aware that typical street photography does not require model releases, because as i have been told there is no "expectation of privacy" on public streets, But due to the nature of these particular photos in which I am establishing a more formal interaction between myself and the person than would someone shooting candid images, I wonder if this would necessitate having the strangers sign a model release, and if so would a so called Pocket model release, as found here http://asmp.org/tutorials/pocket-model-release.html be sufficient for my protection?