When you hit the street with the idea of photographing people, your mind has to be faster than the shutter release of your camera.
Personally, I do not like persons being aware that they will become the subject of my shot. There are some considerations to support this choice:
1) If people notice that they being photographed they will change from natural mood to a pose.
2) They will turn their head away to avoid the lens.
3) They will cover their face or even scold you on your intention to photograph them.
All of these will kill a candid street shot. So, try to shoot from a distance with your longer lens and use shutter speed to freeze expressions and avoid motion blur.