Not always a lot, I don't think. I know a few people who make a real point out of having "good posture" because they've heard that it makes them look confident and/or better. On the other hand, I try to make a big deal out of not having what I see as "strutting good posture" because 1) I don't think artificially "good" posture is always attractive, 2) I don't want to look more "strutty" than I really am, 3) I'm not in the Miss America pageant, and 4) I like to create the impression that I'm not as confident as I am because I find too much "strutting confidence" in a person unappealing. So, at least in my own case, what my posture should tell someone is that I'm not about to take on an artificially stiff posture just because people say it creates a better impression.
I like people to be natural and not contrived for one reason or another. That means that sometimes my posture is fine, but I'm not above slumping in my chair if I feel like it. I suppose if I were either someone who wanted to fake confidence because people say others should fake confidence, or if I were someone who in fact were overly-confident, I might opt for always having great posture and a confident air. I'm neither. I really find "head stuck in the air" and "shoulders back" unappealing.
The day I enter either the military, the "Super-Model Business", or the Miss America pageant will be the day I worry about having perfect posture. Since that day isn't going to happen, I'm happy to be "just me" and not worry about effecting phony body language because someone says it's better to "look confident" than to just be me.
The way I see it, there's something very confident about the person who is relaxed enough to slump or sure enough of himself to feel free to look at the fallen acorns or interesting patterns in the sidewalk as he walks (rather than worry about who thinks what about his posture). LOL