Kecia
Admin - CPST Instructor
But it also makes sense to me that if you yank the car seat with both hands and the car moves but the car seat doesn't, that you may have overtightened the belt to the point where, over time, it can weaken the seat belt and potentially put undue stress on the belt path of the child restraint, as well as damaging your upholstery.
See, this is one of those things that (I'm guessing) you heard from one of your instructors and they probably heard it from one of their instructors and so it keeps going year after year. But I'm going to challenge you on this one(totally in a nice, friendly way) and ask you to back up this statement with something from a credible source. Are any of these concerns addressed anywhere in the student/tech manual? Have you seen them addressed or noted as a concern in a CR manual or a vehicle manual? A CEU presentation from a manufacturer rep? I haven't, but that doesn't mean it's not out there.
Now I certainly don't claim to be a webbing expert either but my understanding has always been that webbing will only stretch and subsequently weaken under extreme dynamic crash forces. And that nothing we can do with our bare hands and little tricks (like reclining and then uprighting the vehicle seat) can stretch or damage belt webbing or compromise the CRs ability to perform as intended in a crash.
I agree that it's not necessary to get it rock-the-car-solid if the seat doesn't want to install that way easily. As long as it's less than 1" from side-to-side and front-to-back, that's good enough. But to state that there is some possible detriment to a rock-solid install is questionable.