Even the definition of those extremes is relative. In a photographic print "black" is actually the darkest tone we can render with a particular paper (and toner) which reveals no detail within its area. Similarly, "white" is any area which contains no silver and we see only the paper base showing through the emulsion. A look at an artist's supply catalog reveals that in painting black and white are also relative terms. There is Zinc White and Titanium White. There is Mars Black and Lamp Black. Each has its own visual characteristics and its own characteristic in mixing the paint.
Some practitioners of black and white photography get very passionate about getting the whitest whites and the blackest blacks possible. It is a quest that sometimes leads to an obsession with using the paper with the "whitest" base and techniques of exposure, processing and toning which produce the "blackest" blacks. All of this may be lost on the viewer if the print is viewed under anything less than optimal lighting conditions.
Good clean whites and rich dark blacks undoubtedly add to the impact of the image by grabbing the viewer's eye but beyond the outline of their area, they tell us nothing about the subject. Areas of solid black and solid white define outlines or shapes but never reveal texture or tonality. They give no sense of depth or atmosphere. All the information that reveals the subject is in shades of grey and it is that information that holds (or fails to hold) the viewer's interest after the initial impact of the image has faded.
Interestingly, the eye does not normally see pure blacks and whites. We see only about a three degree angle sharply and the eye's iris constantly adjusts to allow us to see detail in the area it is focused upon. We see the folds in the whitest fabric and the texture of the blackest charred wood. Only when it is overwhelmed by a specular highlight or an unusually dark shadow surrounded in brightness does the eye register "pure" white or black.
The most important element of the image is the content. What are you trying to say? While the accents of black and white are important, some images will speak most clearly with no pure black, others with no pure white, a few with neither. Make the content the primary force in your images. The content alone should determine the vocabulary of tonality in your prints. When printing, don't neglect the blacks and whites, but don't become obsessed by them. They are the salt and pepper, not the whole dish. If you wish to make images which speak clearly to the viewer, become a master of grey.
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"One of the symptoms of an approaching nervous breakdown is
the belief that one's work is terribly important." Bertrand Russell