I would never tie down my machine at any point above the suspension. Unless you tighten the straps down enough to pretty much bottom out the suspension, you run the risk of the machine shock loading the straps and breaking them if you hit a big bump. Leaving the suspension heavily and continually compressed for a long trip isn't good for the suspension.
Ideally, I would like to have tie down points right next to each wheel and secure the wheel to that point.
Currently, I hook my straps to the top part of the wheels and run the straps from the front wheel to the rear of the truck and the rear wheel to the front of the truck. The body of the ATV moves around as I am driving down the road, but the wheels stay locked into place. In my F350, I don't feel the shifting weight of the ATV, but your mileage may vary if you have a lighter truck.
I check tension on the straps after 5 miles and again after 10 miles, then at every stop.
- bsd