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.