One thing you don't want is movement-when I used to have to transport curved glass on the truck at the glass shop I worked at, I would clamp it down tight in the middle against the rack, then put a little pressure against the ends so they couldn't move. If it starts to move around (and flex) that's obviously when it's going to break. Same as laying it flat on the ends, or with the ends up, if it can flex it's toast.
and yeah, the PPG (sorry, PGW) vans that delivered our glass had racks like those in them
