As long as they're welded all round, there should be ample bending stiffness in the corners (they'd tip before they lozenged) but I've seen commercially produced ramps that are barely tacked together, more reliant on their form (triangular ramp and trapezium sides so they stack).
If they're homebrew, I imagine they'll be more than adequate as they'll be assmebled with a lot more care, but for a couple of feet of flat on each section, diagonals would be cheap insurance.
