The only thing that I wouldn't do is change the character of an answer. If an example is consistent with the answer and contributes to bettering answering the question, I might add it, I might comment that an example would help, or I might answer the question myself if I felt that the example was the most important thing or the answer would be significantly different with an example (e.g., the example would need elucidating).
I wouldn't add an example that wasn't consistent with the answer, though. If an answer talked about foo, bar, and baz, I wouldn't add an example that used widgets and gadgets. I also wouldn't add an example to an incorrect answer to demonstrate that it's incorrect. In that case I would use my own answer, with an example, to show the correct solution.
My opinion is that, while this site has some wiki-like characteristics, it's not like Wikipedia in that we aren't trying to formulate a single, correct answer. If that were the case there would only ever be one answer to a question and we would all just contribute to it. Rather we each have our own answers -- or ones we agree with that we can vote up -- and the wiki-characteristic that is in play is editing and amplifying to improve clarity, not to change content.