People seem to think that if your saddle rolls, you are not a good rider, not riding balanced, etc. but honestly, after trying lots of different saddles and trying to find one to fit my Mustang, the horse's conformation plays a BIG roll in how secure your saddle is.
Round backed horses are notoriously hard to fit a saddle securely on, and you really have to have a well fitting saddle to feel your saddle is secure on one.
On the other extreme is my Foxtrotter mare. Because she has a good set of withers, I can throw just about any saddle on her and it will be secure. That doesn't necessarily mean it's a good fit on her, but she has withers that will hold almost any saddle. I am frequently checking the cinch when I ride my Mustang, but I can ride with a really loose cinch on the Foxtrotter.
But no matter what the conformation of the horse, a well fitting saddle will be more secure than a poor fitting one.
I find flex trees to be less secure than regular trees.
I don't know exactly what Parelli recommends for saddle placement, but I DO think that the farther back a saddle is, the more likely it is to roll.
I guess what I am saying is, before we all start to jump to conclusions that someone is a poor rider because the saddle rolls, you have to take into account the conformation of the horse, the fit of the saddle, and the circumstances of the accident.
I have had a saddle roll with me twice in my 16 years or so of riding. I consider myself a decent rider. But sometimes "life happens," especially when riding out on the trail. It doesn't mean necessarily someone is a poor rider. Your horse's conformation and fit of the saddle can make a major difference in how you ride out a spook or buck.
I guess I sort of got off topic, but frequently when people speak of saddles rolling and such, the first response often is "well they must not have good balance because I can ride with a loose cinch." If you think it can't happen to you because you are a good rider and doing everything right, then you just haven't met the right circumstances yet!