I also have never used a fence in front of a horse. I used to show trail horses, but that was not why I wanted better control of the movement and the position of the horse.
I probably used my horses' ability to position themselves more for handling gates and moving into position to handle young horses from the saddle on a broke horse than for anything else. I just was always very particular about how a horse moved laterally and how a horse moved its shoulders.
I want a horse to ALWAYS cross his front foot across in front of the other front foot as he yields over. You can only teach this correctness if the horse is moving forward as he moves over. Then, when you start using less 'forward' and more 'over', the habit is set to always cross over correctly.
I am also very picky about straightness. I want as little reverse bend as possible. I don't want a horse's nose pointing to the leg he is yielding from if at all possible. This comes from my desire to control a horse's hips and from wanting them to eventually learn how to do a correct half pass and flying changes on a straight line.
So, if you stuck with all of this, the bottom line is that if you start with 'leg yielding' exercises and get them done with as little reverse bend as possible, you are setting your horse up to do a correct side-pass the very first time you ask for one.