right there on page 4 is a pretty picture explaining exactly what propagation delay and transition time are. Propagation delay is the time from the midpoint of the input transition to the midpoint of the output transition. I, personally, would design for worst case values. If you design for typical values, you're just asking for trouble.
Without further information about your circuit it's not possible to offer any more advice. But I have to ask why you would cascade these gates. Two inverters is exactly the same as no inverters.