It depends on the shock/springs you have on the car.
If you are running GM Shocks/Springs even the ZOK, it won't be an issue.
With Lower springs or coil overs, In most cases, you will contact the muffler before hitting the valence, but it kind of depends on the drop off and how crappy the roads are in your area.
It's really no lower than the stock black Valence. Instead of having the fins drop down lower than the stock valence, he removes the space between the fins.
I hope that makes sense.
Kelu is the only person that I know has a documented case of smashing his, but that is because he runs KWs and doesn't know how to tune his suspension.

{See how long it takes for him to see this}