The law of inverse squares applies only to light coming from the source because of its spread. The farther the source is from the subject, the less light that falls on the subject. Once it falls on the subject, and is reflected, exposure remains constant regardless of camera (or reflective meter) distance.
With a 20°, or even a 5° reflective meter, one needs to be aware of what it is measuring, however. In most cases, a meter like that will be measuring both highlight and shadow values, and will give an averaged reading. That may be OK, or it might be misleading, depending on the subject's contrast, lighting ratios, etc. Personally, I prefer to take reflective readings with a 1° spot meter, so I can check the values for smaller areas, and thus keep all values within the capabilities of the film (or, the contrast range required by the presentation media, such as a magazine ad).
Another key factor to keep in mind with all of this is that metering and exposure is all relative. That is to say, the values in the scene, and thus the exposure, are relative to each other. Thus, by lighting elements in the scene separately, one can adjust or manipulate the relative values, making them fall within the desired exposure and contrast range. That's what we're doing when we add "fill" (either with a reflector or a flash) to an outdoor portrait. That's also what we're doing by lighting a background separately from the main subject. If one can control the lighting, and the relative exposure values, a light-colored background can be made to appear much darker, or a dark background appear to be much lighter.