Hmmm, interesting. I do have my fronts filled and have had absolutely no ill effects. My 990 owner's manual makes no statement one way or the other whether or not to fill the fronts with liquid ballast. It does talk about the suitcase weights that are available to hang on the front (as I'm sure all tractor manuals do).
It is obviously OK to add weight to the front, so what possibly difference could there be to the driveline whether that weight is hanging off the front as cast iron weights or lives inside the tires as liquid??? Either way you are giving the fronts more bite and more traction. In fact, the suitcase weights should theoretically be more harmfull because their weight is felt by the front wheel bearings where liquid ballast isn't. Also, what about adding or removing a loader - that will make way more difference to the traction of the front end than having filled tires or not.
Personally, I'm not the least bit worried about damaging anything. As far as steering control, I expect the only time it could possibly be negatively affected is at high speeds (heavier wheels having stronger gyroscopic effects), but I don't think our machines go fast enough to really make any difference. I certainly haven't noticed any difference at all between loaded and unloaded tires running down the road at top speed (and I bomb around at full speed quite a bit).
There's my $0.02 /forums/images/graemlins/tongue.gif