When John Jones is listed on the 1850 census with a wife named Mary, and in 1860 with a wife named Elizabeth, don't assume it's the same woman and name her "Mary Elizabeth." Never add a middle name as a way of reconciling two contradictory records!
Just because a man was named Charles Griswold Austin (as in my family), that doesn't mean his grandson Charles Austin had the same middle name.
Inappropriate middle names can mislead other researchers. One of my ancestors had sons named John and William (among others). I was surprised yesterday when I found a cousin calling John "John William." I reviewed all my sources--two censuses, marriage record, military record, death record, and will, and found only the name John. Not even a middle initial. Then I looked at FindAGrave and there he was called John William (no sources, as usual), even though the grave stone photo showed only John. Multiple Ancestry trees had picked up the name John William, and because of that had mixed up many of the records for John with records for his brother William.
So, please--just use what the records say, and resist adding undocumented middle names to profiles!
Edited to add minor clarification.