The general rule I use is that if I have a primary source (e.g., a name mentioned in an ancestor's will) or reliable secondary source (e.g., a name mentioned in a census record or church birth record) that provides a name and a general time frame as to when/where the person lived, I will add them to Wikitree.
Here is a good example of this - my grandmother, Barbara Borkovic, apparently had a brother named George:
http://www.wikitree.com/wiki/Borkovic-8
He is listed in a secondary source, as there is a baptismal record in St. Nicholas Church in Millvale, Allegheny, Pennsylvania for George Borkovic, the son of Anton Borkovic and Mary Kralj, born February 21, 1903.
I have not found any other records mentioning George, and his name does not appear in the Pennsylvania Death Records, 1906-1924 or in his father's 1907 estate record. As such, he most likely died as a very young child before 1906.
Even though I only have one source for George, I chose to add him to Wikitree - in hopes that someone else might come across another record mentioning him.