I find something immensely sexy about the attitude that causes a woman to modify part of her body to look the way she desires rather than leaving it however it happened to look before. That being said, while that attitude is attractive to me, I tend to gravitate towards women who not only have that attitude, but also have a taste in expressing it which I find aesthetically pleasing.
I take umbrage at the idea that it's foolish to decide to get a tattoo first, then decide what. It depends on what the tattoo is for. Some people get a tattoo to remind them of some important event or person or idea in their life. That's fine. Some people, however simply want to commission a piece of art from an artist. I think a woman who has the taste to recognize the talent of an excellent tattoo artist and the stones to pay that artist to use some portion of her body as a canvas for creating whatever the artist desires is sexy. I've seen many tattoos that have obviously been done by a person with real artistic talent. In some cases, the talent only came through because the artist was allowed to express himself however he wanted. I think there's something admirable in allowing that to take place.