The dreaded "chair leg". I fight it constantly. I think this was the best I ever got with bringing my leg under me:
I did a lot of dressage, but I did more jumping, and jumpers need their legs to be a slightly more forward than the typical dressage position.
If you're not trying to win a beauty contest, ride the way you want.
However, the reason for the "proper" position isn't just because of balance. I have seen a lot of people who have their leg back, but the rest of their body is teeter-tottering and unbalanced. The leg should be back because this is the general area we use for our aids. Or theoretically, the "proper" area that our leg aids should be used for.
It helps us move the barrel of the horse and control the hind end when our leg is back where its supposed to be. Leg position is equally important in reiners, especially since the majority of their direction comes from the seat and leg aids. Are there rules about position for reiners? Not really. But its a bonus if your leg is back where its supposed to be so you can communicate with your horse directly instead of having to swing it back everytime you need to say something.
7 years later and I look like this:
Dreaded chair leg. Maybe one day I'll overcome it. But it probably won't be today. Tomorrow doesn't look good either.