Never button the bottom button on 1) single-breasted suit jackets, blazers, odd jackets, etc.; 2) vests; 3) overcoats.
For nearly 100 years, men's businesswear has been cut based on the specific assumption that you will not button the bottom button. What started as a fad now undergirds our entire enterprise.
You may consider buttoning the overcoat's bottom button if it's really damn cold and precipitating, but your scarf had better coordinate immaculately.
If you decide to button the bottom button on a suit jacket, I'll be expecting a 1,000-word essay from you 24 hours in advance explaining why.