If you're editing in order to provide sources for facts already stated in the answer, do it. That's one of the reasons we allow editing: to improve answers and make them more complete.
If you're editing to provide new information, you might be better off just creating a separate answer.
Just a reminder to everyone reading this: you have no reason to expect that your posts will remain exactly as you wrote them, in perpetuity. When you post anything on Stack Exchange, you agree to our Terms of Service, which explicitly state that everything posted here is licensed by Creative Commons (which says that other people can modify your work), and our FAQ also lets you know that other people can edit your stuff:
All contributions are licensed under Creative Commons and this site is
collaboratively edited, like Wikipedia. If you see something that
needs improvement, click edit and help us make it so!
All edits are tracked in a public revision history. To view revisions,
click the edit date on the post.
If you are not comfortable with the idea of your contributions being
collaboratively edited by other trusted users, this may not be the
site for you.
If someone edits incorrect information into a post, or deviates from the original intent of the person who posted the question or answer, the post owner can roll back the revision and the community can adjust their votes (each time the post is edited, you are allowed to change any vote you may have cast).
If you want to add a comment asking if the OP minds you adding in information, that's fine for the sake of politeness, but it is not at all required.