I'm not a hundred percent on anything.  But I don't see why you couldn't where the shirt either way. In my opinion it gives two different looks and depending on the pants or skirt and if you plan on using a belt it might look better one way or the other. *shrug* I've never tucked a shirt in in my lifetime, even under a blazer. I think it's preference and like I said the look you want. The shirt out looks slightly more casual. Whereas in with a belt or high-waisted pants or skirt gives a more sleek/professional look. Just use your best judgment and where what you think looks best for the situation. Certainly there has to be more important criteria than whether one tucks in their shirt or not. 

I've always "lived" by those rules, but then I still live by the rules that bra straps should never show, a shirt should never hang below your sweater, a jacket should come to at least your waist, and you should never show belly skin except when wearing a bathing suit.   Obviously, I have very out of date fashion rules, so you probably shouldn't listen to me. 

I'd say it depended on where you were going to wear the shirt.  I'm no fashonista  but often I will tuck in a shirt just in case.  Even though I'm so high-waisted it makes me look like I'm wearing my pants all the way up to my neck. 

I'm not a fashionista.  That's kind of difficult to be when I generally think that women over 70 dress the nicest and to the standard that the rest of us should be dressing to.  I am aware of those rules and I'm also quite happy for anyone who breaks fashion rules.