This needs investigation, as I'm not sure how wide the options are in google earth. But right now we take the centroid of the line, put a point there that has the label, even if it's not actually on the line. And we put a big old placemark icon there, which looks like crap - when you're zoomed out you just see a ton of points, no lines.
I believe we should be able to just put a label on the line. The place where it may break down is being able to click on the placemark and get all the attribute details. If we just put a label on the line we might not be able to do that - as that's definitely the case in polygons. So if people want attributes enabled perhaps we could find a better algorithm for the placement of the attribute point, and also something that makes the default not such a big point.