As Frank has posted, there are usually innumerable variations of spelling of any name. I think the accepted consensus is to try and use the spelling the person themselves used (for example they may have signed a document that is cited as a primary source.) barring that the earliest record (usually a birth or baptismal) is used. The earliest record statement also applies when a person has personally used different spellings throughout their life (I have one individual who during his life signed his name as Sproul, Sproule, and Sprowl.)
I personally avoid relying on Census records for spellings if possible and merely state as part of the source what surname the individual is listed under if it is not obvious (for example Groulx instead of Sproul)
When you are merging you may not have access to the sources each person used (if any) - in this case I would use the earliest source and add a note to the bio such as "The surname of this individual has been cited as Gerard, Gerhard, and Gerrard; Gerard has been adopted as the Last Name at Birth because it is the earlist documented spelling. <ref>Birth Registration</ref>", in additoin to listing the alternate spellings in the a.k.a. if appropriate. If you run into a situation where there are two different transriptions of a birth record and the original document is not available I would favour the one that follows the spelling of an immediate family member that has a documented spelling (Parent, Sibling, Child) and mention in the bio that the spelling is disputed.
