I usually shoot at whatever distance I can get the area of the subject I want to portray in the shot depending on what focal length I'm using, if It's a head and shoulders I usually on 35mm use an 85mm lens and work at around five feet.
Moving farther from the subject and using a telephoto lens provides enhanced isolation of the subject against the background; the reverse is true if the photographer moves in closer with a standard lens, all that background clutter will be included. The effect that results is entirely up to the photographer but unobtrusive, complimentary backgrounds have always been popular in fashion/modelling.
I'm all over the map on this one depending on who I'm shooting and what the desired results are. I've been successful from extremely close to the other side of the room with a variety of lenses. One thing that I always do is communicate to the subject to keep them informed. If I feel I need to be extremely close I will ask first which tends to put them at ease.
In my studio 99% of all portraits are done at a distance of 1.5 to 2 metres. Why?
In "Western" style societies this is the distance that two strangers set when they are engaged, interested, attentive, respectful, but not invasive of personal space. This gap is so familiar and consistent that facial features, ratio of nose to ears, chin to neck, etc, just look "right".
Once the distance is known framing is organised by choosing the appropriate focal length; long focus for tight face portraits, wide angle for half-figure, and so on.
Photography, the word itself, invented and defined by its author Sir John.F.W.Herschel, 14 March 1839 at the Royal Society, Somerset House, London. Quote "...Photography or the application of the Chemical rays of light to the purpose of pictorial representation,..". unquote.