I don't think your asking will actually "upset" someone. I think that when a stranger asks personal questions, they become naturally defensive or protective. I would think that some initial reaction might be your are selling or stealing. Making them feel comfortable with you would be my first priority.
I think the email or snail mail suggested by Frank is a good idea. If you are using email, you might provide a link to a page on wikitree explaining what your project is. I believe people feel more comfortable when they see it is publicly available on a reputable site.
Also, I would include some information as to how you got their name.