I was going to do this but with the sandy soil at my house the dealer thought compaction might be an issue when I mow. I showed him that I have 4 - 50 pound wheel weights and they let me buy it w/o fluid or the ballast box (I'll also be running either a box blade or back back and am going to mount 5-42 lb weights if I really need to get agressive with the FEL).
The manual says you need 500-700 lbs out back for big lifts, so fluid alone if probably not enough. I noticed with w/o any weight the back is pretty light just with the FEL on the tractor, so ballast is mandatory.
Fluid is a good choice, however, if you do this many people recommend tubes to avoid leaks and corrosion on the inside of the rear wheels.