I agree with Helen, and take an even simpler approach. No matter how offensive someone is or how unacceptable the comment, I smile and say, "Thank you."
If they were trying to be funny, even clumsily, no harm. If they intended to be constructive, even poorly, they think you understand. If they were trying to be offensive, they realize you aren't going to get sucked into it and there really is not an actual response. And if once one says "Thank you," they persist, a pleasantly mild smile (though certainly not a grin) straight into the face, in silence, usually results in them falling silent or in someone else putting a bit of pressure on them, and without adding to the total unpleasantness of the world.
I am lucky enough I do not have to perform, but love it that this works with overbearing supervisors, evangelists, vicious gossips, rowdy children, and even those folks who, when you tell them you are taking a trip, feel called upon to tell you about every plane wreck they ever heard about.