The post about using "Why would I want to do that?" with a nurse got me thinking about when each of the standard phrases would be best used.
I came up with:
"Why would I want to do that" for when someone is asking for something that obviously benefits them a lot more than you. In other words, the kind of request that can't possibly be rationalized and you want to make that clear. It may not work if the person asking has a whole laundry list of reasons why you should, indeed, do things her way
"Will you take Little Munchkin to school every school day before your commute, while she eats yogurt in the back of your car, for me so that I can get my hair done?"
"No is a complete answer" for those times when there might be good reasons for you to do something but you have your own reasons not to want to do it, which you do not want to discuss. It is hard to discuss something when the other person doesn't give arguments to be refuted. "When will you quit smoking?"
"I am afraid that won't be possible" is the light version of "no is a complete answer", to be used with clients, customers or others where a simple "no" may seem curt.
"Complete silence" is for when the question is completely out of bounds and you want to leave that clear to the person. The sort of question where even giving a "no is a complete answer" is too much information. For example, "Is your DH going to have a vasectomy?"
"So kind of you to take an interest" is the lighter version of "complete silence". It is also useful in situations where complete silence may be interpreted as not hearing the question.
"What an interesting assumption" is best used when the problem is not a question but an affirmation that needs to be questioned in a polite way. "You chose your hair color to match mine."
Bean dip is a generic tactic, usable in almost any situation.