A person can go overboard on saddle fitting. I like this book and often recommend it: Amazon
.com: The Horse's Pain-Free Back and..."> Amazon.com: The Horse's Pain-Free Back and...
However, it makes it sound like a custom saddle, professionally fitted several times a year is the only hope.
A lot of weight goes on a small area, so getting a saddle that matches the horse's back is good. Obsessing on it is not.
My gelding had an ill-fitting saddle put on him and it cut a hole into his withers, but he was being ridden daily, all day, cutting and roping cattle (and at 800 lbs, he's too small for that anyways). Very few horses get worked 8-12 hours/day, every day.
My horses may be ridden about 3 hours/week each. At that rate, I'm not inclined to buy a custom saddle. You can check the sweat stains after a ride. If everything is even, odds are the saddle is close enough. My gelding is a bit odd for having such long, tall withers combined with a very short back. In fact, it is hard to find saddle pads that don't cover both shoulder and hip at the same time, and a western saddle that is right in the front will usually go too far back. Square corner ones will hit his hip. English saddles don't have that problem, but it is hard to find one that isn't too wide horizontally.
Happily, he's a good natured horse who isn't ridden hard. Compared to ranch life, he's barely used at all. I just watch for signs of irritation - emotion, not physical - then check to see if I can make it better.