Well, I mounted mine horizontally and I cut the bottom inch off the rear fender in the hope that it won't get snagged by the rear tire at full shock compression.
In Dirt Rider's very favorable review of the FE570 a couple years ago, they made mention of the problem of the rear tire snagging the rear fender. Apparently the tire will catch the fender, bend it under, and then break it. That sounds like a pretty bad experience to me. Dirt Rider indicated that they heated up the plastic fender and straighted it's curve so that it remained out of the way of the tire, and they said they had no further problems with it. I've read that the fender may partially return to its former curve.
At any rate, that's why I didn't mount the license plate vertically.