Hi Stacey, and welcome to Wikitree,
In general the birth record, or in it's absence the earliest possible record should be used, literally the Last Name at Birth. Some people do not 'like' this guideline as all the children of a family might end up with a different variation, but the community needs to have an objective standard that can be applied when different lines of descendants who have adopted different spelling variants cannot resolve which spelling should be used for their shared ancestor.
The real challenge can be deciding what form a patronymic "surname" should take (Should the son of Peter be given the "surname" Peters, Petersz, Peterse, Petersen, Petering, Peterink, etc....) often this involves looking at the conventions of their time/place, and what they were known as in later records.
With all this said, some 'common sense' also applies - if all prior records for the parents, and all later records for a child use one particular spelling and the birth record is truly a one-off then the 'common' spelling should be used with an explanation in the biography.
Ultimately its up to the profile contributors to decide what name variant to use - the only time the guideline is likely to be 'enforced' is when two contributors have a dispute about the surname that they cannot resolve.