Take a measurement of how long your cars are that you'll be hauling, figure the added space needed for proper tie downs, then give the trailer maker your measurements where you want them. That's what I did. Trailer makers will put them anyplace you want them. I took a measurement off of my Galaxie, even though it wasn't on the road then. I figured it would be the longest vehicle I'd ever haul. I figured out where the best place was for the car, for proper loading, then figured out where the D-rings should go. I added a 3rd ring at the front, in the middle, to use an offset winch, which is really nice to keep it out of the way. You can see the middle ring in this shot, although it was before I added the winch against the wall.

If you do add any D-rings, just make sure they're reinforced with a plate on the outside. I also put mine where it straddled one of the center rails from the tongue, so I think I could pull just about anything without worry of pulling through the floor.
Dale, Rory and Don gave me tons of good advice while I was researching on what options to get on mine. It saved me lots of headaches of realizing what I should have gotten before it was too late. I'd stay away from the vinyl faced walling because every one I've seen has the vinyl peeling or shrinking and starting to come loose. I also saved a bunch of weight and money by doing the floor myself with a 2 part epoxy floor paint with flakes added for traction.