Guys - just sharing experience from my pain..if towing over a long distance..best to use a strap that holds the wheels to the trailer. lets the body/suspension move without affecting the tie down straps.
Now granted, I was on a 1600 mile trip, but I had a strap over the two front rails to the floor which failed during day 2..allowing my car to roll forward and backward in the (luckily enclosed) trailer when I stopped/started, buggering the trunk handle, denting the 'boot lid', and some rear light damage.
My next trip, I ante'd up for some over-the-wheel straps..she doesn't move an inch now..
$20 at US cargo control..