[Cool Hand Lurker]

I have found that using carpet pieces for padding where the tie-down strap goes across sharp edges can sometimes move, especially when the strap is a little loose from the load shifting. I use a short one foot section of 2" PVC lay-flat discharge hose and slide it over the strap before tying anything down. It is soft and resists kinking and twisting. It protects the car and keeps the strap from chafing or cutting. ( Really important! )
I keep a couple of them on my 2" tie-down straps and just slide them along the strap to where they are needed.
You can buy this stuff by the foot at Menards or Home Depot or any farm supply store.