Here's my take based on my cars and my experience with what's needed
whole hood on my m3, m5, gallardo, r8, cgt, f430. After 1 broken windshield on the R8 and a nasty gouge on the hood, this is an absolute must for the Audi.
nothing on the hood on my wife's Range Rover Supercharge. The truck just sits too high imo to need anything up there. On the Q7, however, I didn't and I'm starting to see chips on the edge. No marks on the RR after 7k miles.
E60 M5 Silverstone, Balloon White Gallardo Spider, Rosso Corsa F430, Audi CG Q7 Sportline, IS R8 before and DG R8 now, Basalt Black Carrera GT, SG M3 DCT, Orange GT3RS, IS CLK63 Black Series
On order: 2010 R8 V10