Do you have to ground a generator?

Not really necessary but can't hurt if you are going to set it up with a transfer switch and backfeed your house.

I guess there is some slim chance a short on the generator could ground to the chassis and present a shock hazard but that is pretty rare.

For the record, I've never grounded a portable generator of any kind.

I believe this is one of these things that is never really done in practice.

Now, if I was using it in a classified environment to power a pump that was moving a flammable liquid in a desert climate? Then yes, I would ground it.