Here is the rule:
If the top you are wearing could be mistaken for a mini-dress WHILE ON YOUR BODY, tights = ok as emergency ass-cover.
If the top you are wearing can not be in any way construed as a mini-dress and is mostly a long top WHILE ON YOUR BODY, tights = unacceptible.
I don't care if you borrowed your friend's tunic and it is plenty long on her, if you feel like you would have to take the tiniest steps in an enclosed, windless space in order for nobody to see your business in only the top, you should really put some pants on. If you could take long strides and maybe flash side-thigh action, but not really any butt cheek and definitely no cooter, tights or pants will do.
I promise I'm working on a better post. It's about handwriting. Get ready!