We're getting ready for a roadtrip with Razz (the wonder dog!) and am thinking I should buy a harness that can be teathered to one of the tie-downs in back. Read an article about what happens to dogs in a serious accident. Not pretty...for the dog or the owners. You consider that?
You should absolutely get a harness for your dog. We have one each of this for our two dogs:
http://amzn.com/B000MD3NOK. We haven't taken them in the X yet, but we always tethered them to the upper car seat tether anchors. Now that I think about it we may need a longer tether since the anchor is so low on the backs of the Model X seats compared to our Accord.
While tethered our dogs can turn in complete circles which they like to do without getting tangled up. I think the harness came with something to attach them to the seat belt, but this was no good as they tended to get tangled, and when our younger dog was a pup she chewed through a seat belt

Attaching to the car seat tethers has worked wonders, and it also stops them from being able to get as far forward in the vehicle.
We plan on putting ours in the third row with a hammock style seat cover. That way we can still store some luggage in the back, toddler in the middle seat middle row, and probably luggage in the outer second row seats as well.