They're everywhere although they really shouldn't be. Humans do tend to lend scale however. Where they go in the frame is contextual usually. To convey a feeling, and to give a sense of scale, there needs to be a relationship of people to the environment around them in the frame. If placement is wrong, it can be misleading- but this may be done intentionally. I tend to make the final, precise decisions about this after the technical work is done- just before pressing the shutter. I kind of go blurry and see everything at once. Context is really about seeing through the viewers' eyes and removing everything they aren't getting that you are, like all the time you had before the shot is taken to see the entire space and all the actions that led up to the moment of the photograph. If you can remove those, then you can anticipate the "naive" eye of the future viewer of the image.