I see this idea offered up often with the expectation grass is less damaging than pavement.
One, you are still going to bend up the belly skins. Two, unless you have driven those grassy areas, you have no idea what is hiding out there. Holes, manholes, storm drains, culverts, you name it. They are not nearly as smooth as you think. Three, you have know made it harder to recover your aircraft, because there may not be a solid surface to work with.
Remember, in an emergency the goal is not to save the airplane, it's to protect the occupants. A gear up landing on pavement is a non event. Worst that will happen is you twist your ankle getting out of the plane. On grass there is the possibility of digging in , pitchpoling over, or cartwheeling. I'd go for pavement anytime.