Untucked, my Tee Shirts are really fitted in general (I've got half decent tone and wear a size small tee for comfort) and the favourite one I have? hmm not sure which one counts as that.

As for Shirt shirts like tie and collar shirts. I wear those also untucked in summer I generally wear one open with the sleeves rolled up over Tees instead of jackets (for extra pockets of course) other than that I wear them untucked with the sleeves rolled up because thats how I've got used to shirts being worn. Even for work I'll wear them rolled up.