I wouldn't worry so much about backing height as much as I would the actual trap placement. I've had nothing more than small clump of grass as the focal point but placed my trap 3-4' feet away because it was the "natural path of approach." I've also placed a trap a few inches from a backing that was 3 feet tall. I do like to keep it 100% natural though. If you go to importing a bunch of stuff, they can and will notice...

Some of the other guys bring up great points on animal personalities, competition, population dynamics, family structure, etc. It's really all subjective...

I made one set a couple of years ago on a trail through the woods during the winter. The backing was a small piece of broken tree and behind that was natural brush. It wasn't real thick either. I put a cell camera up quite a ways away so I didn't have to be in the area to check. For some reason the coyote always worked the set from behind and no matter what the wind was. I really never got a good idea why they were doing that.