Hm. I've been married more than half my life and I have no idea how the spouse 'tucks'....
There's a topic for discussion: 'Which way do you tuck your penis in?'
As for t-shirts, those are his 'work clothes'; he's never worn an undershirt, but I'd have to assume the same thing Mal said, it keeps them neatly flat.
(personally, when wearing camisoles, I don't tuck them in-they don't reach the top of my undies
)