You're right - the engine will be forced up due to an impact. Most manufacturer's have this type of design in their hydraulic rams for just that purpose. Whether a bump will cause the engine to lift, or not, I do not know for sure. If there were no hydraulics - sure - I can definitely see that happening. But with the hydraulics pulling the engine down under a lot of pressure, I "think" there will be no 'lift-off'. If there is, it is probably imperceivable. However, it can't hurt to tie it down, either. If you do that, though, a bungee isn't going to offer any help. Even tieing a rope won't help much. To do it right, you would want to use a ratchet strap.
A good way to get a better feeling on this would be to simply try and lift up on the engine while it's on the Wedge. This obviously isn't a definitive test as we (well, at least me!) can't supply the same amount of force that a bump on the road can. But if you can lift it, then a bump certainly can. I suspect (and this is just a guess), that quickly lifting up on the engine is simply going to lift the whole boat.
I believe this is why the manufacturers recommend (for those that have a strong enough trailering bracket) to not just rest the engine on the bracket, but to actually snug the engine down onto the bracket with the hydraulics - to keep it firmly planted due to the immense pressure that the hydraulics can provide.