About leaving the bottom button open on single breasted suit, there is the following great lessons by The Art of Manliness‘ Art of Manliness Suit School-Part III: A Primer on Suit Buttons:
1) One Button Suits: One button suits are the easiest to remember. The button should always be buttoned when standing and unfastened when one sits down.
2) Two Button Suits: Two button suits are also simple. The top button should remain buttoned, while the bottom button is left undone.
3) Three Button Suits: With three buttons, you have options. You can either button the top two and leave the bottom unfastened, or simply button the center button.
April. It is the season when new employees who wrap the body in a new suit overflow in the town. I often see that a young man buttons all buttons on single breasted suit without knowing the basic suit manners. Each time, I feel like teaching him how to button suit buttons. Yes I was like the young man when I was a new employee. On the other hand, it is every once in a while but there is a time when I see a man old enough to know the basic buttons all the buttons of single brested suits. I will not say anything about it.