As for editing other people's posts - I believe edits should not deface, misinterpret or paraphrase entirely what has been mentioned by another person. An edit makes sense when the answer has broken links, horrible grammar, formatting problems and other reasons that may prevent the readers to better understand and apply it. Usually the review system of the Stack Exchange sites allows for reviewers to flag invalid, too minor or destructive edits appropriately.
That being said, your edit might have all the good intentions, but it could never make it live due to the review system itself. Apart from that, the author of the post might disagree with your suggestion and your edit could become a reason for frustration, and series of comments like "hey, I never had the intention of posting this, revert it please".
A new answer will avoid the above issues. I'd personally write my own answer, in case it will significantly differ from the existing one in the following cases:
- I am adding extra related information that the other answer omits
- I am adding specific information for potential problems that may be encountered during the application of the solution, and list at least one work-around
- Address the issue with respect to the OP specific scenario (usually I write comments to the question first to get details if not present). Of course, the existing answer has to miss the points I am willing to add, or have them too obscured.
- I had the same issue and I feel confident at properly addressing it.
In other words, if the answer can be of value to he OP or users with similar problems, then it is good for the community to share your experience and expertise. If the answer is not that good, the community will react accordingly anyway, so obviously this is a good-enough criteria if you doubt whether "to answer or not to answer" - if it will be good for the community then do answer :)
In cases where the answer is close to yours, and the differences or improvements are insignificant enough not to write a separate answer, you could address your concerns as comments. If the author does not reflect them, they will still be available as a comment, and as such will add to the answer.