proper western posture

If you are truly going to be riding endurance I believe that the lengthy rides will adjust your legs to be under you, and you will ride more balanced. It isn't a matter of showing or what is popular. Riding shoulder/hip/heel makes it easier to ride, period. Bracing your feet in the stirrups helps you stop easily, but you are riding behind the motion if you go from a halt to a full gallop, something that QH's can do very well. (Arabs can do that, too--"Corporal" loved to take off, when asked.)
Don't care if you do, or you don't. I got in the habit and I trained myself out of a chair seat many years ago. I kept glancing down at my foot, and would pull it back under me any time my toe was poking forward. Granted, riding English, it's easier to post when your feet are underneath you.
After the first 10 miles of a race, you may find yourself dropping the stirrups and re-taking them many times, from fatigue.

JB, for what it is worth, yes you are in a chair seat but I hardly paid any attention because the background of the pic is amazing! Where the heck is that? I'm soooo jealous!

I ride in a wintec dressage - I do my distance conditioning and LD in it. If you know someone who has a WD that you can try, give it a go - let your stirrups out so that you can JUST put your heels down (barely) and you'll be able to get a really good feel of where you need to be.

How do you look with your big horn? Is it any better? From your amazing awesome cool background photo (LOL) you could probably get by with dropping your stirrups down two holes - see where that gets you.

It's in City of Rocks, Almo, Idaho - the AERC Championship is going to be held there this year It was by far my favorite ride. I felt like I'd stepped into the LOtR movies or something! And I didn't even see some of the prettiest parts! It would have been my favorite ride with just the terrain, but on top of that there were hot spring pools with showers 5 minutes down the road! Best. Ride. Ever.

I'm not riding at the moment (weather/wanting someone to come check my saddle for fit/need to focus on some ground work), but I'll post a picture ASAP. I already know it's too big for my butt (I'm getting a cover to help with that) and that I need to drop my stirrups down, so I'll post some pictures once I make those adjustments.

I stopped reading at page 7, but I will actually say that I agree with BSMS for once.

I ride with a decent SHH alignment, maybe my heel a touch forward but only by an inch or so.
But when I ride with too-long stirrups and my feet are far forward, I can definitely feel that my back is SO much more loose and fluid with the horse than if I am in my normal position. I can't sit a trot to save my life with my usual stirrups, but I can with the "chair seat" and my horse will open up and move very freely.

I am hoping to start endurance soon and maybe I can find a happy medium within my position.