Suit Coat Buttons - Both buttons buttoned on a two-button suit?

Several of my female friends are under the impression that a two-button suit is properly worn with both buttons buttoned, and that the bottom button being left undone starts with three-button suits. I’m a fairly conservative dresser and have always thought that, while having the bottom button buttoned trims the line of the suit, it’s one of those silly rules, stemming from a fatass British royal, that you just have to follow.

(I should point out, for the record, that the gentleman with both buttons buttoned has a fan club, so it’s possible that it’s just okay with them because he’s a fabulous guy to begin with.)

I’m familiar with the “SANs style” rule for three-button suits, but can two-button suits be properly worn with both buttons buttoned?

Also, what’s the proper etiquette regarding one-button formalwear? I seem to recall seeing a lot of one-button classic tuxedos, and if I remember correctly, the morning coat I wore to prom (however incorrect it is to wear morningwear to an evening event, it looked damn good) was single-buttoned. I’m assuming two- and three-button formalwear follows the same rules as regular suits.

Set me right or help me nod smugly, please.

My general rule is:

If 1 button, button it.
If 2 buttons, button 2 of them.
If 3…
I unbutton them when sitting down, and button back up with one hand as part of the motion of standing.

The Casey School of Thought:
Single breasted, one button. Double breasted, two buttons.