Oh, you have got plenty of extra weight then, 100# per rear wheel is Great. On (or in) the wheels and low is excellent.
If you do fill the tires, a product called "Rim-Guard" (google it) is the best thing, safe for rims and the earth.
If your tilling, you should have some weight on the front for balance and steering, even for plowing, a little extra weight on the front wheels will help with steering.