Hmm. Yeah, I'll admit that if I'm really tired or sleepy and not 100% sure of my balance, I'll sit down to use the facilities in the middle of the night.
The rest of the time, though, I definitely stand.
As far as the whole seat up vs. seat down thing goes, I often put it down when I leave simply because we have three females in the house (wife and two daughters) and it only seems courteous.
I've been known to leave it up now and then, and when I do, my older daughter can be heard through the entire house shrieking, "Somebody left the seat up again!" Then she'll stomp out, shaking her finger, and accost me with great ire - "Daddy! You left the seat up again."
My typical response varies.
Sometimes I use, "Well, when you start flushing it, I'll start putting the seat down."
When I'm feeling less playful, I go for something more blunt. "Ok, fine. I'll leave the lid on my toilet up, and you can leave the lid on your toilet down. Oh, wait a minute. I almost forgot. This is my house. All the toilets belong to me!"
Josh