Historically, yes, the hemlines went up during the war (WWII) because they were conserving fabric.
As for what's different "these days" than the minis that girls wore in the 60s and 80s, is just what 33 said - if you wore short on the bottom, you wore long on top. The ideal non-skank coverage rule - according to Cosmo - is 40% skin showing. So if you're showing your legs, no cleavage. If you're showing off the girls, wear a longer skirt, leggings/tights/opaque hose or pants!!
I personally can't do the tea-length look, or even below the knee for that matter, because I am super stumpy. At 5 1 3/4" and short legs, my waist isn't high enough to make the look work. So while the trends may be going towards flowy, tea-length dresses, I will have to stick with my tried and true shift dress silhouette.
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