Let's say someone asks a question that requires multiple "parts" to an answer (for example, an advantages/disadvantages question). Now let's assume someone answers this question and touches on most points, but is missing a point or two (and let's say his/her answer was picked by the OP as the correct answer). Would it be better to add that additional information as your own answer, or suggest it to the author of the correct answer in a comment? Also, if the answer is a community wiki, is it good etiquette to edit another person's answer and add more info?
If you have information that further answers the original question, I would submit the information as another answer (instead of adding a comment). That way it comes under the scrutiny of the voting system and allows users to comment directly to your answer.
Also, the general etiquette of editing posts seems to lean more towards copy editing; making the post more clear and concise. Correcting grammatical problems or spelling errors is okay. Adding to or changing its meaning is not.
Robert Cartaino's answer is somewhat ambiguous to me. I agree that if you have additional information that significantly answers the question you should add another answer, but I also think that you should include all of the other information so that your answer is comprehensive.
Every answer should stand alone. They might cite other answers, but they should answer the question as completely as possible.
The only general exception to comprehensiveness should be in cases where there are actually multiple independent answers. Tho, even then, the best answer would be one that exhaustively and definitively includes all possible answers (and any important info about the differences thereof).