I think that schools are looking to see how well you've thought through the process. It's a "fit" question in my opinion. If you're applying to Tuck and you tell them "Tuck is my top choice, I'm also applying to Chicago, Wharton and Columbia" they're probably gonna have some questions about how well you understand the culture up in New Hampshire. If you tell them "I'm applying to Tuck, Cornell, Kellogg and Stanford" then I think they'll be more comfortable with the idea that you're picking them because you're looking for a certain type of culture.
Obviously it depends on the school, but schools that aren't all about fit probably won't ask this question.
If a school is really your top choice I'd tell them why. If a school isn't I would say that I'm looking for schools that fit a certain profile, I have it narrowed down to a few, but I don't have a definite order right now, explain what the profile you were looking for is and then explain why their school is a good fit.