For me it's 'Leggings must be worn with something that COVERS YOUR BUM'
I'm sorry, even with teenagers with lovely pert derrieres, leggings and top that stops at your waist look like you haven't got dressed properly, and on those of us with less than gravity-defying butts (such as myself) it just looks horribly unflattering, especially if you wear cheapass leggings where the fabric goes all see-through when stretched.
What rules do you stick to, even if out of fashion?