I've tried on the rear tire, next to the body and the front bumper. I've gotten the best results as far as SWR goes on the front bumper given all other factors the same. My only guess is that the close proximity of the body was messing with it somehow. But I know nothing about the workings so that's just a guess. But it was the only way I got the SWR close to 1.x. next to the body was close to 2.5.
Edit:
As an additional benefit it makes a great clearance indicator on the front bumper. If it hits anything on the trail I know I need to watch my clearance.