After three years on WikiTree, these are the guidelines I have developed for myself:
1. I do not create profiles for living people, ever, unless I have their express permission.
2. I do not create profiles for recently deceased (with still living immediate family) except if the profile can be created with truly publicly available information (e.g., a Google or Bing search would reveal the information).
3. I do not create a profile unless I have a credible source for birth year and place (at at least the state/province level) and a current intention to continue to research that person and add additional information to the profile.
Otherwise, I include the information I have in the biography, such as names of children, possible parents and siblings, etc. (FYI, the text of biographies is fully searchable). I also, every so often, do "maintenance" on the profiles I have created and check to see if people I have included in the text have had profiles added to WikiTree by others. Sometimes they have been, and then I link them up as appropriate.
With these guidelines, I am comfortable I am growing a healthy WikiTree. Others probably have developed other guidelines that work for them.