I think that in cases where the question is meant to collect a sorted list of responses (or other questions that "should have been community wiki to begin with"), like the example you provide, the answers should be community wiki for reasons discussed elsewhere.
In cases where the question got converted to community wiki automatically due to a large number of edits, but is still a mathematical question with a definite answer, I see no reason to automatically convert the answers, and doing so may cause people to feel that they are being deprived of reputation that they 'earned' by providing a helpful, informative answer.
Once a question has been converted to community wiki, previous answers remain wiki or not depending on how they were marked when written.
If you see a question that should be made community wiki, you can flag it "for moderator attention," so that a moderator can convert the question+answers if appropriate. You can also comment on the answers so that the people who wrote them can change them to wiki. The first is likely to be faster.