ORIGINAL: SJAdventures
The reasoning they gave for disgarding brass after 3 trimmings is that the case neck thickness will be getting pretty thin by then. I would have to check again but I think I read this in my Lyman's 48 edition.
You can check that believe it or not. But I too like to just toss most magnum loads after 5 firings. And standard rifle loads like 7. After a person has a ruptured case, they think twice before pushing thier brass to the distance.
If you don't knock back the headspace, you will not have case wall thinning like you will if you FL size everytime. Thats the reason I generally try to use collet dies or bushing dies.