Suppose you come across a community wiki answer and feel you have something to expand/change/correct. What is the accepted etiquette for editing community wiki answers? Is there a strong sense of "ownership", or are we free to "make like a wiki"?
This is exactly the initial objective of the Community Wiki.
There is a lesser sense of ownership, the initial poster's name signature disappears, replaced by the name of the person who contributed the most, and showing the percentage of contributed. Also, users with 100+ reputation will be able to edit them without having to go through the peer-review system.
It is really made for everyone to contribute and keep the post updated, so yes, you should feel free to add what you want, as long as it helps making it a better answer/question.