Well, that depends. Are you one of the "zomg Mooneys can't land on grass" crowd, or are you merely concerned about short field performance?
If you've never landed it on grass before, just go somewhere and do it a few times.
If you're concerned about performance... Well, so am I. I have an R.
@German guy and
@Skywalker have an E and have been in and out of 6Y9. Same wing, but their gross weight is 2575 pounds, mine is 3368. Me sitting in the pilot's seat with no luggage and no fuel weighs more than their max gross. Accordingly, I need to develop more airspeed to get the wheels out of the grass, and to climb above the trees, which are rather large on the west end. And I like to bring my wife, son, luggage, and fuel so we're probably going to be close to gross. Even just enough fuel to fly down to KLNL legally, on a standard day from a paved runway, my ground roll is 1500 feet, and 2500 to clear a 50-foot obstacle. There is no published non-paved performance chart or fudge factors (except that "high humidity can add 10%"), so I'm kind of in no-man's land.
I am probably going to try to fly the R in this year anyway, but I'll be trying a few takeoffs without the family and luggage first, and know my abort point (and pay attention to it) Hopefully I won't need to get the plane home on the back of a truck! I do have a slightly better power loading than the E (12.0 lb/hp vs. 12.9), but my wing loading is much worse (19.4 lb/sqft vs. 15.4 for the E).