Are you asking this to have it included in the FAQ, or because you see a need for it (i.e. it is more of a feature request)? In case you're interested this very question has been asked many times before.
To answer your specific statement: in general answers should be amended to either add more information or to correct small issues - you shouldn't be changing an answer simply because you don't agree with it.
Further to that, you have to decide why you don't agree with it:
- is it simply the writing or answering style?
- is it factually incorrect?
- does it answer the question that was asked or is it tangential or right off topic?
- do you disagree with an opinion stated in the answer?
If it is simply the writing style that you don't like, then leave it alone, you don't have to cast any sort of vote for it (in fact the absense of a vote is a penalty in itself because only answers with merit should be upvoted).
If the answer contains factual inaccuracies, are they easily fixed? If so then fix them. If not then think about leaving a comment. There have been a few times in the past where I have downvoted and then left a comment stating that if xyz gets fixed I will reverse the downvote. However a comment for downvotes is not necessary and never has been on the SE network.
If you simply don't agree with an opinion stated in the answer then it is your right to up or down vote accordingly. Try not to leave a comment about it, the SE sites are question and answer sites not discussion forums. If someone includes opinion in their answer then it should be clearly discernable as opinion rather then fact, and they should be aware that they may collect up or down votes based on the opinion stated.