"If you need your weight in the saddle to prevent the back of your saddle from popping up, then your saddle doesn't fit."
If you tied a board to a table at one end, and then bounced the other end of the table up and down, that end would flap. It would fit perfectly, but the motion of the table would toss the loose end up and down.
If in doubt, I'd rather have a saddle tree with too much "rock" than too little. If it curves too much, I lose some of the weight distribution capability. If it bridges, then I create pressure points at both ends. The former is less harmful than the latter.
After several months comparing different trees and forms on Mia, I had a saddle made for her using the best fitting one (based on both my observations and the pictures I sent "SouthernTrails", and after discussing backs and trees with the head of the Steele Saddle Tree LLC. This is how it looked if she ate grass with the saddle on:
This is how it looked when I rode her:
Bandit has a flatter back than Mia. Ideally, I'd buy another saddle for him. But I can't afford a custom saddle for every horse. In fact, he is such a slender horse that a perfect fit would probably require a custom saddle tree to go with the custom saddle. But since the saddle seems to stay on him if I mount with the cinch loose, and has stayed on him during several spooks with a cinch that was fairly loose, and since he doesn't act bothered, I figure it is close enough. When I tried him with my Australian saddle, it DID seem to bother him after an hour or so - so the Aussie saddle is retired for now.
The important thing from my perspective is that the saddle stays put even if the front cinch is looser than I like and with no rear cinch. My weight will create even pressure, not a second cinch in the rear. I don't want to test it, but I think my saddle would stay on him if my cinch fell off - as my daughter's did with Trooper.
BTW - some of my comments are based on seeing a video where a respected saddle tree maker compared a rocking chair on a floor to a saddle. He said the rear cinch was needed at all times to prevent the rocking chair from rocking. I think the answer is to both get a better fitting saddle, and to put some weight on the saddle - like a human. The loin will always move more than the withers, so more motion will always be applied to the back end of the saddle - even with a tree that fits well. But you do not use a rear cinch to strap down the rear end of a rocking chair on a table-top horse's back - I'm pretty sure we all agree on that!
When someone ropes cattle, it is a different story. I can't speak to hills, since the ones I'm willing to ride up or down don't stress things the way some of our more adventurous members do when riding in places that would make me wet my pants just to look at! I also know some folks who use a rear cinch and breastcollar every time they ride, although they don't ride anything more adventurous than me. They just like the look & feel and their horse seems happy so I'm happy for them. I think they LOOK cool, but it is just more tack to carry to me...