Excellent topic.
I have spent many years researching the tan line debate. Also, countless discussions with friends and the girls I have dated.
My results....
1. Skinny women who suffer from a flat ass should sport the thong tan line. It deceives the eye into a more shapely ass.
2. Larger woman with narrow hips should also go with the thong tan line. It gives the appearance of a smaller stomach and more shapely hips.
2a. Larger woman with narrow hips and large breast should go with the above mentioned thong tan line. But, should also go with the bikini top tan line. It creates the illusion of a more defined breast area that is more balanced with her hips.
So, My opinion on tan lines depend on the woman's body type. Just as it does for my opinion on how a woman should groom her most private of areas.