This is a tough one for me, as I had a really bad experience with a saddle slipping when I first got back into riding as an adult. It was my first canter lesson, I was on the lunge, and as the little Haflinger I was riding picked up speed on the circle, she remained on the circle while the forces of nature and the loose girth caused the saddle to slip off to the right, and threw me head first into the wall.
So now, I'll admit it, I'm a chronic overtightener. I'm so embarrassed, but I gave my poor mare girth sores from overdoing a fleece-lined girth; the fleece compressed after only a few weeks of using the girth, and the stiff material underneath was rubbing her raw.
After a couple of weeks off for healing, I switched to this girth, which seems to be a lot better for both of us. The elastic is on the inside, so I find that while I can get it very snug to begin with, I can't overtighten it. As we ride, it does stretch, and by the time I dismount, it looks borderline loose to me, but she seems much more comfortable now. And, no sores.