I think it's a mixed thing. There is something kind of sexy about tan lines. ((To be clear I'm particularly talking about bikini tan lines, not a full one piece, which would be weird.)) The biggest issue is that you can't be dark bronze and have really pale lines all the time. A permanent tan line bikini isn't really all that sexy. But a light tan with tan lines can be. It's a very hard thing to explain exactly. Maybe it has to do with the fact that the tan lines "high light", no pun intend, the specific areas that they do.
I say cheesy. If a girl is soo tan, that you can REALLY notice those white strap lines, its kinda funny. Especially if she's wearing a bikini, and her boobs have a white triangle and the rest is dark. Hmmm.