John, you are right about the manual. No one really seems to know the reason, but some of the speculations are mentioned above in this thread: some sort of damage to the steering or 4wd or hopping. On the other hand, I have read quite a few members reporting here and on CTB that they have filled front tires with no noticeable bad effects.
Personally, I wouldnt worry about filling them on my 2910, and if I did so, I would use the Arnco Superflex foam because that will give the most fill weight (about 65 lbs per tire) and also flatproof the tires. I have not done so yet because 130 lbs. of fill isnt very much, and I can probably jury rig something with weights on the the front end or grill guard that will be both heavier and removeable.