Wikitree rather disappointingly assumes that there is a definitive ie. correct spelling for each surname
asked by David Mortimer
Not at all. It is not that there is an assumption that there is only one correct way to spell a name. What we are told to do is use the earliest known spelling from documents, and to place variant spellings in the other last names field.
This may not be easy when the earliest document lists the same name with more than one spelling in the same document, but each person needs a unique identifier - and Wikitree, as does FamilySearch and other sites do, uses the earliest known last name as the last name at birth.
Variant names aren't that different from determining the correct patronymic when documents may also use soldier names.
If Samuel Byers uses that spelling, but his children use Biers, then he gets Byers, they get Byers LNAB and Biers CLN. Then their children get Biers LNAB. Any other spelling variations go under OLN.
If Joseph Smith decides halfway through his life he wants Smyth, then before he dies he changes it back, you explain it in the biography, while using Smith as LNAB and Smyth as OLN.
edited to correct an oopsy.