I think it is good form to avoid replying to an insult with an insult. We should always be the better person and rise about the situation, but often is it difficult to ignore the temptation and ease to go even lower than the original offending statement.
The people who ask the more stupid/insulting questions are more often that not so oblivious to their comments, they are beyond even the simplest forms of reason. Leaving them baffled by a cleaver retort is your best option. You leave looking like the thoughtful man you are and they just end up having their stupidity confirmed.