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MWThere are many ways of looking at the situation, but I believe that a "master set" should contain one of every nationally distributed card in any given set and one of every card that was part of the regular or similar production process (yes, in some cases this may include one-of-a kind type cards that were produced in contest oriented issues).
A standard set of say, 1951 Topps Current All-Stars or 1923 Maple Crispettes, may only include those cards that were widely distributed, but in order for the standard of "completeness" to be perfectly satisfied, a collector must possess one of each card that is attainable or possible. In these two cases, that means the 1951 Stanky, Roberts and Konstanty as well as the 1923 Casey Stengel (even though there are only one or two known copies).
However, I do not believe that "proof cards" or "printing errors" (not production errors or printing changes) would be part of a master set.
Reasonability may also be a consideration. In some cases where multiple colors or production brand names exist, it may not be reasonable to own one of each card.
Of course, this is just my opinion. Ultimately, I think the final determination lay with each individual collector.