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...